a sexrch for r-parity violx~inc; at hera …...i hwe to especially niention my house mates, dr....
TRANSCRIPT
A SEXRCH FOR R-PARITY VIOLX~INC; SCPERSYMMETRIC P . ~ ~ I C * L E S AT HERA r:srh-G THE ZEUS DETECTOR
Raphnel Galea
A t hesis submit tecl in conforniity wit h the reqriirenierits for the degree of Doctor of Philosophy
Graduate Depart nient of P hysics Cniversity of Toronto
Copyright @ 2001 by Raphael Galea
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Abstract
Dedicat ion
For Nannu Ki1 le.
Acknowledgement s
Experimental High Energy physics is a collaborative effort and I miist thank an rnorrnoiis
number of people. For this reason. should a name be missirig in thesc acknowledgements.
it will certainly be found iri Appendix C which contains the author list for t h e ZEUS
collaboration.
My thanks must first go to rny aclvisor Dr. John Martin. wliosr iirifailirig support.
has been present throughoiit ni- stiidies. 1 am also gratefiil for t h r giiitlaricr o f Dr.
David Bailey and Dr. 'rlike Luke. and for the encouragenierit I receivecl froni Dr. Sarripa
Bhadra. Thank you. Dr. Bob Orr. for introdiicing rntl to the graduate program.
In Germany 1 learned a great deal througb collaboration with the past and prrsrnt
coordinat ors of the Esotics and Rare Phenornena Ptiysics groiip: Dr. Strfan Sdilenstcdt .
Dr. Masahiro Kuze. Dr. Roberto Sacchi. Dr. Liica Stanco and Dr. Briictl Straiit). I
worked closely with Dr. Stefan Polenz and benefited h m liis e s p e r t i s ~ ancl fricndship.
Thanks also to the other Canadian group Researcli Associat~s Dr. Bill Schmidkr. Dr.
Garry Levman and Dr. Gerd Hartner. Dr. Ernrnaniielle Perez deserves a great dcal of
credit for her pioneering nork at H l on t tiis topic a n d hcr important coiitrihiitions to t l i t b
HERA Monte Carlo workshop.
Thanks must also go to the secretariat at DES\'. Fraii Bahr ancl Frau Brrtzkc I aiii
also grateful to Winnie Kam and Maianne Iihurana a t the University of Toronto Pliysics
Department for their guidance and help throughtout m - time in the School of Gradiiate
Studies.
The Italian group at DESY were like faniil? to nie. I hwe to especially niention my
house mates, Dr. Salvatore DePasquale. Dr. Stefano ':Vostro Huomo' Dusini. Dr. Sicola
'Cippolone' Coppola and of course Vincenzo. whose nickname is unprintable. Alberto.
Silvia, Polini, Maria Carrnela. Antonio: Enrico, Marta, Daniella, Davide and the rest of
their compatriots never let me forget where I was born. There is life outside DESY ancl
riiy friends Sonja. Kristina, Carsten. Pet ra. Dominik. Icla. GiGiLs and the Spanish c r o d
helped me to esperience it.
1 have to also thank Andrea 'THE OX'. my Friend. house mate. and meniber of the
Canadian group. Vielen Dank für alles. 1 must not forget to acknowledgc the entirr
Canadian ZECS cluster. it [vas truly a group which reflected Canada's cultural divcrsity.
!darce Pat. Laurel. Slohsen. Peter. hlichael. Sanjay ancl Tliorsten. ['II nevcr forgct the
'good old da YS'.
To Christian. Steve. Ali. Daniel. Dr. Pepptir and Sneaky Dees. we started antl aliriost
finished together. The 'Heir Apparent'. Etiennc. and cspecially Amit helpetl rtir to
remember to take tirne For myself.
Thanks to Gina and .\lison for continually fiw-tuning the Feiig-shiii in oiir l ioiis~
until this thesis was completc.
And finally I must thank niy parents for proviclirig rntx with il11 t h r rii3ccssary charar-
teristics to succeed and enjoy my choices in life. Equally iiriportant to riiy sl1rct.s~ i w s
rny sister. Serena. whose support. emotionally financially antl otherwise has nlivays twrn
unyielding.
FI-ahhar
Contributions t o the ZEUS Experiment
I arrived at DESY to begin my work a t the ZEUS experiment in September 1996. Cpori
arriva1 1 worked on the Third Level Trigger (TLT). 1 becanie responsible for the niain-
tenance and upgrade of the software and hardware for the TLT dong with a Researcli
Associate (RA) from the Cniversity of Toronto. 1 sas on cal1 during data takirig For ;in-
online problems relating to the TLT. In the ivinter sliutdown between i996 and 1997.
1 Iielped in integrating a new architecture into the system and 1 performed a major rc-
organization of the monitor and filter software code. Oïer the years niore deniaiid lias
been made on the TLT in terms of processing poiver. Along ni th the RA. 1 portetl rnariy
offline data quality checks to the TLT.
In 1997. 1 managed the Canadian clustcr cornprised of several platforms and opcrating
systems. That same year 1 joinecl the Exotic and R i i r ~ Pherionirna physics groiip. Ii i
1998. 1 became the group's trigger representative. Along with riiy trigger responsibilit ios
for the Exotics group. 1 initiated the Data Quality llonitoring (DQSI) for the ~ s o t k
physics triggers. In 1998. the DQM consisted of the Event of the W e k (EOT\V) projrrt.
in which the liighest energv most interesting events arc selected and scanrit'tl. In 1099.
I implemented a simpler version of the EOTK selectiori code onlirie. a t ttit. TLT. iir i<i
created the Event of the Day (EOTD). ivliich perfornis a real tiriie csotic selectiori. This
allows the two person shiftcrew an added visual opportunity to look a t the data. ivliic4i
has made spotting potential detector problems easicr.
1 have represented the ZECS and H l collaborations at two international conferenccs.
DIS99[1] and LLCVI2000[2]. In 1998 1 perticipated in the year long HER-\IIC[3! worksIiop
in which 1 worked on generators of R-parity violating supersyrnrnetry at HERX.
Finally. 1 participüted and assisted in a meeting of the ZEUS collaboration in Toronto
in Junet 2000. 1 designed and managed the meeting web page and addressed an- com-
puting concerns of our collaborators.
Contents
1 Introduction
2 The Standard Mode1 4
. . . . . . . . . . . . . . . . . . . 2.1 Basic Elements of the Stanciarct llodel 4
. . . . . . . . . . . . . . . . . . . . . . . . . '2.1.1 Generation of mass 6
. . . . . . . . . . . . . . . . . . . . . 2.2 P rob iemswi th thcS tandard~~odc l 6
Phenornenology of Supersymmetry 8
3.1 The Mnimal Supersymrnetric extension to the Standani Mode1 (.\ISS.\I) 11
. . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Seutraiino Sector 12
. . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2 Chargino Sector 1:)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 R-Parity 14
3.3 Phenornenology of Supersymmetry at HERA . . . . . . . . . . . . . . . . 16
3.3.1 Resonant prodiiction of Squarks . . . . . . . . . . . . . . . . . . . 17
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Squark Decap 21
. . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 P, Squark Decays 21
3 3 . . . . . . . . . . . . 3.4.2 R-parity conserving or Gauge Squark Decays -- 3.5 Neutralino and Chargino Decays . . . . . . . . . . . . . . . . . . . . . . . 25
3.5.1 XeutralinoDecays . . . . . . . . . . . . . . . . . . . . . . . . . . 26
vii
. . 3 . 2 Chargino Decays . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 i
3.6 Possible Final State Topologies . . . . . . . . . . . . . . . . . . . . . . . 29
3.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4 The Experimental Setup 33
4.1 H E R k Hadron Elektron Ring Anlage . . . . . . . . . . . . . . . . . . . . 33
4.2 The ZEUS Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . :3G
4.2 . 1 Tlie Central Tracking Detector . . . . . . . . . . . . . . . . . . . 3;
4 . 2 . The Uranium Calorinieter . . . . . . . . . . . . . . . . . . . . . . 39
4.2.3 Other Components . . . . . . . . . . . . . . . . . . . . . . . . . . 43
. . . . . . . . . . . . . . . . . . . . . . . 4.2.4 The Luminosity hlonitor 4 1
4.3 Trigger and Data .A cqiiisition System . . . . . . . . . . . . . . . . . . . . 45
4.3.1 Thr First and Second Levd Trigger . . . . . . . . . . . . . . . . 45
. . . . . . . . . . . . . . . . . . . . . . . 4.3.0 Ttie Third Level Trigger 47
HERA Kinematics and Event Reconstruction 59
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Phpics at HERA 39
. . . . . . . . . . . . . . . . . . . 5.1.1 Deep Inelastic Scattering (DIS) 59
. 5 1 2 Reconstruction of Kinernatical Cariablcs . . . . . . . . . . . . . . 6.3
. . . . . . . . . . . . . . . . . 5.1 -3 Reconstruction of Global Quantities 63
5 . 2 Track and Vertex Reconstruction . . . . . . . . . . . . . . . . . . . . . . 65
3.3 Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.4 Electron/Positron Finding . . . . . . . . . . . . . . . . . . . . . . . . . . 63
- - 5.3 Jet Finding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
. . . . . . . . . . . . . . . . . . . . . . . . . 6 Squark Mass Reconstruction 74
a.6.1 Channels mith a e" in the final state . . . . . . . . . . . . . . . . 74
5 - 6 2 Channels with a v in the final state . . . . . . . . . . . . . . . . . 14
viii
6 Monte Car10 Simulation 76
6.1 Background Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
6.2 SCSY Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
6.3 Detector Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
7 Event Selection 85
7.1 Search Strategv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
. 7 2 Ciit Optiniization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
7.3 NC-like . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
7.3.1 Preselectiori . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
- i 3 . 2 Further '1'C Seiection . . . . . . . . . . . . . . . . . . . . . . . . . 91
7.3.3 Jet Cuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
7.3.4 c jet Sample 97 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.5 e je ts Sarnple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
7.3.6 Selection Efficiencies and Squark S I i s Resol~t~ions . . . . . . . . 100
7.3.7 Final NC-like Event Sarnples . . . . . . . . . . . . . . . . . . . . . 100
7.4 CC-like . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
. 4 Selection cuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
7.4.3 Signal Enhancernent . . . . . . . . . . . . . . . . . . . . . . . . . 109
8 Limit Setting Procedure 116
8.1 Inclusion of systematic uncertainties . . . . . . . . . . . . . . . . . . . . 117
8.2 Interpolation and limits in the SCSY phase space . . . . . . . . . . . . . 118
8.3 Systematic Vncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
9 Results 123
9.1 Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
9.2 Other experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11'6
10 Conclusion 130
A CC-like Cleaning Cuts 132
-1. l Calorimeter Timing cuts . . . . . . . . . . . . . . . . . . . . . . . , . . . LX
-1.2 S C positron/clectron criteria . . . . . . . . . . . . . . . . . . . . . . . . 1 3 3
B Branching Ratios and Signal Efficiencies 134
C The ZEUS Collaboration 139
D Glossary 150
Bibliograp hy 153
List of Tables
2.1 The fermions of the Standard Mociel .
. . . . . . . . . . . . . . . . . . . . . 3
2.2 The bosons of the Standard 'Llocicll . . . . . . . . . . . . . . . . . . . . . . .
3.1 Particle content of 1LISSXI . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.2 Squark production processes at HERA via 61, coiipiings with a positron
beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.3 Squark dec- channels in @, SCSY which worp c*onsi<lereti . . . . . . . . 31
3.4 Squark Decay Charinels in J?, SCSY rlassifitbci hy distirirt topologirs . . . . 32
4.1 HERA design and 199'7 runriing parameters . . . . . . . . . . . . . . . . . 34
.. 6.1 Sumrnary of generated background 'ilonte Carlo saniples . . . . . . . . . . r l
6.2 Summary of generated signal Monte Cürlo saniples wit h niiiltijet f i n ci 1 statcs . 80
6.3 Sumrnary of generated signal lIonte Carlo samples a i th single jet final
states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
7.1 Summary of data sample event numbers and espectation from Monte Carlo
background simulations for events with a reconstructed e= . . . . . . . . . 103
2 Summary of the cleaning and selection cuts for the CC DIS sample . . . . 108
7.3 Summary of data saniple event numbers and espectation from Monte Carlo
backgroundsimulationsforevenrs w i t h & . . . . . . . . . . . . . . . . . . 114
. . . . . . . . . . . . . . . 8.1 Summary of systematic uncertainties considered 1'2'2
List of Figures
3.1 Radiative corrections to the Higgs n ias iri a siipersyninietric niotiel. . . .
3.2 The cvolution of the SG(3). SC(2). and L7(1) couplings in the (a) the
Staridard SIodel and (b ) t hr Mininial Siiprrsyninietric Extension to the
Standard 'LlodcL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Neutralino masses giwn at fixcd ( a ) Jb = LOO Ge\' (b) -\I? = 200 Ge\'
and tan 3 = 3 as a function of p. . . . . . . . . . . . . . . . . . . . . . .
3.4 Chargino masses given at fiseci (a) JIz = LOO Gr\* (b ) -1- = 200 Ge\' arid
t a n 3 = 2 as a function of p. . . . . . . . . . . . . . . . . . . . . . . .
3.5 Decay of a Proton via LQD and L-DB. . . . . . . . . . . . . . . .
3.6 Interaction vcrtices via A:,, couplings. . . . . . . . . . . . . . . . . . . . .
3.7 Production cross sections for C L and & for A' = 0.1 in e p collisions. . . .
3.5 Squark &, production and decay diagrams. . . . . . . . . . . . . . . . . .
3.9 Regions in the SCSY phase space (Jly. p ) corrcsponding to a charged LSP
and a neutral LSP dominated by its 5 . 2 or f io cornponerits. for tari 3 = 2.
3.10 Dominant gauge decay mode for a 150 Ge\. irt sqiiark in the phase space
plane ( J I 2 . p ) for tan 3 = 1. . . . . . . . . . . . . . . . . . . . . . . .
3.11 Representative diagrams from the 4d, decay of the f: for a non-zero Xi,,.
3.12 Gauge Decays of the t:. . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.13 Gauge decay modes for the Fz. . . . . . . . . . . . . . . . . . . . . . . .
S..
Xlll
4.1 Layout of the HERA accelerator complex and injection system . . . . . .
-4 . 2 The integrated luminosity recorcied by HERA for 1992-2000 . . . . . . . .
4.3 Cutaway view of the ZEUS detector . . . . . . . . . . . . . . . . . . . . .
4.4 One octant of the CTD showing the field and sense wires . . . . . . . . . .
4.5 The ZEUS calorimeter (4-2 projection) . . . . . . . . . . . . . . . . . . . .
4.6 The Layout of the ZEUS luniinosity rnorlitor . . . . . . . . . . . . . . . . .
4.7 Schematic diagram of the ZEUS trigger and data acquisition susteni . . .
4.8 The TLT hardware design . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9 The TLT software design
4.10 Flow chart outlining the TLT trigger decision . . . . . . . . . . . . . . . .
4.1 1 Time measurements for ep and beam-gas iriterüctioris . . . . . . . . . . . .
4-13 Tirne distributions of RCAL tinitis versus thc FCAL ttiiriirs RCAL tirrit .. .
4.13 Examples of cosmic and halo miions in the ZECS (irtwtor . . . . . . . . .
-1.14 The CPC processing time required by t h r TLT . . . . . . . . . . . . . . .
5.1 A schematic view of the Deep Inelastic Scattering Process ;it HERA . . .
5.2 A schematic view of the s-channel resoriuncr! production at HERA . . . . .
5.3 Display of a charged current SIonte Carlo r v m t . . . . . . . . . . . . . . .
4 I J J ~ as compared to gregulorlrcd for a liigh Q2 charged ciirrerit Slontc Carlo
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . sample
. . 3.3 The positron finding efficiency as a function OF energ y. polar angle arid
6.1 Examples of leading order diagrams for (a) direct and (b ) resolved photo-
production processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2 Graphical riew of the gerierated signal Nonte Carlo sets in the gaugino
mass space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
siv
Effect of initial and final state parton showers on the mas reconstruction
of a 200 GeV squark undergoing the 4(, decay (i + eq. . . . . . . . . . . . -
o(eTd _t as a functiori of the SLSY pürarneter p for fixed .\- and
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . fised squark mass.
.Jet distributions for Monte Carlo sig~ial sirnulations with e= and mii1tiji.t
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . topologies.
.Jet clistributions for Monte Car10 iieiitral and chargetl ciirrmt hackgroiind
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . simulations.
.Jet distributions for hfonte Carlo signal siniulations for e and single jet
topologies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.Jet Distributions for the pr~sclected data satnpltt. . . . . . . . . . . . . .
Kineniatic distributions for th^ prc!sele(:rrd data ~ i i r ~ i p k . . . . . . . . . .
log,, (Q') and reconstruct~tl n ias distributioris for t h pr twl~ct rd data
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . sample.
The ET distribution of tht. highwt ET jet as a Iiin(*tion of thr invariant
n i a s of t hc e + jet ( s ) susteni. . . . . . . . . . . . . . . . . . . . . . . .
(a) The !j distribution for ncutral current DIS corripard to a sarnplr i:iis-
cade decay (e jets) and a sample 4 decay ( r jet. ( h ) The y distribution
as a function of the invariant m a s . . . . . . . . . . . . . . . . . . . . . .
The log,,(Q2) distribution for rieiitral currerit DIS cornpareci to that of a
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . cascade squark.
7.10 The experimental efficiencics for squark decq-s a5 a hnction of squark mass. 101
7.11 The mass resolution For squark decays as a function of q u a r k mass. . . . 102
7-12 The reconstructed mass distribution for the e jet final sample. . . . . . . 103
7.13 The reconstructed mass distribution for the et jets final sample. . . . . . 10-4
7-14 Distributions of events in the (niimber of good tracks)-(number of al1
tracks) plane. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
7.15 The (a) PT(- l i r ) and (b) f i distributions before the application of thpir
selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
7.16 Control plots comparing data and Monte Carlo backgroiind simulation
espectation for the niimber of good jets. y. Pr arid ET clistribiitions after
preselection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 t 1
7-17 Control plots comparing data and hlorite Car10 backgroiind sirriulatiori
expectation for the logl,(Q2) and invariant m a s distributions after pres-
election . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11'2
(a) The efficiencies and (b) resolutions for cascade sqtiark ciecays into u jets
as a function of the squark miLss. . . . . . . . . . . . . . . . . . . . . . . 114
The reconstriictecl mass distribution for the u .jets tinal sarnplt?. . . . . . . 113
The systernatic uncertainty of variation of the selection cuts or1 thcl sclec-
tion efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1
A;,, lirnits as a function of Jii for f ixd .II2 = 290 G d ' . 11 = -200. 'LOO Gel'
and tan 3 = 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.5
The 95% CL upper limit on A;,, in the SCSY pararrieter (JI2. p ) spacc for
= 200 Gel'. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
95% CL exclusion upper limits on the pl, coupiing Xi,, as a function of
thesquarkmass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127'
Diagram for the selection squark production process at HERA . . . . . . 128
Branching ratios at fised JI2 = 90 Ge\-. p = -200 GeY and tan .j = 2 . . 135
Branching ratios at fised :LI2 = 190 GeV. p = -200 GeV and tan 3 = 2. . 135
B.3 Branching ratios at fixed :LI2 = 240 GeV. p = 100 GeV and tan 3 = 10. . 136
B.4 Branching ratios a t fixed -1f2 = 290 GeV. AL = 200 Ge\' and tari 3 = 10. . 136
5 Efficiency as a function of squark m a s at fised tan .-3 = 2 and p = -200 G ~ 1 * 1 3 i
B.6 Efficiency as a function of squark rnass at fixecl tan 3 = 10 anci I L = 200 Ge\'135
svii
Chapter 1
Introduction
In 1999. 1 presented a prelirninary study of this analysis along a i t h other exotir particlr
searches performed at ZECS [l]. at the Workshop on Deep Inelastic Scattcring in Berlin.
Germany. At the conference dinrier. 1 ivas sittirig a lmg side a st'nior nicnibrr o f o i i r
collaboration. He askecl rnp what the topic of rny tlicsis nu and 1 r d d hini thnt 1 \vas
looking for Supersymnietry. To this 1 received t iicl conmient .*SUS Y (Srrpcrs?/nlrrretvj) t s
the only mode1 which has sumued uuer 25 geears urithout a single piece of eqilîzrriental
proof ".
The point is well taken in the contest of the great acconiplishmcnts by t l i~orists ai ic i
experimentalists in the field of particle phpics;. The Standard lIodel (SM) of particle
physics. which is briefly discussed in chapter 2. has beeri the culmiriation of the successt~s
of the field and has been able to explain a wealth of precise esperimental data. The
discovery of the top quark[4] and firsr direct observation of the tau neutrino[Z] have
filled in the last gaping holes of the SM particle spectrum. with one exception. This is
the important Higgs particle which by most indications seems to be right around the
proverbial corner. Existing measurements have already
to a mass of 108 GeV[6] and the Tevatron run II and
ruled out the Higgs particle up
anticipated experiments at the
planned Large Hadron Collider (LHC) are espected to discover the Higgs particle or
push its mass Iimit to up to the Ievel of 1 Te\*[i].
Despite its siiccess. the SM is not a Theory of Everything (TOE). Ttierc arc esperi-
mental datai81 nhicti suggest that neutrinos Iiave m a s . which is strictly not a part of the
SM. by design. The SSI and some other shortcomiiigs of the SM are outlined in cliaptcr
2. One proposed answer to physics b e o n d the Slf is SLSY. A mininial supersymm~tric
model's most draniatic effect is in cloubling the knonn pürticle spectriim. Each fermion
in the Shf Lias a scalar superpartner and each boson has il fermioiiic superpartrier. SCSI-
searches have been performed at al1 the major esperiirie~its aroiirid the world aiid t t i ~
search for SCSY a t HERA is the topic of ttiis thesis. HERX (sre ctiaptt~r 4) Iias a iiniqiir
opportunit- to test SCSY models involving the concept of R-parity violating (&) super-
symmetric quarks (squarks). The search for & qua rks is promising at HERA bccaiisr
they can be produced right up to the kiriematical limit (300 G d ' ) . This phcnornmology
is described in chapter 3. In the search described iri tliis ttiesis. no signal is obsrrved for
the production of &, squarks in 17.7 of P - p data taken by t h ZECS rsperinicmt
from 1994-1997. Chapters 5 and 6 describc the tools and iiiethods bu rvliich da ta is
reconstructed and in chapter 7 the event selection is esplained. Litiiits ikrr set on ttiv
strength of 4 couplings as a function OF the squark rriass arid parameters whicii clefirie
the models under study. Chapter 8 oiitlines the rnethodology usecl in the setting of the
limits. The results and their discussion can be foiind in &apters 9 and 10.
I can understand the frustration expressed bu niy senior collaborator becaiisc this
SCSY search does not rule out a mode1 but instead orily sets liniits. This is preparatory
work for the upcoming experiments which ail1 Further piish the frontiers of the SII arid
perhaps begin to rule out or confirm SCSY models. To answer the comment regarding
Lack of experimental evidence for SC'S)-. i responded with an anonynious quote: "The
pessimist would say that no SUSY particles have been discovered. The optimist would suy
CHAPTER 1. INTRODUCTION
thut half of them have ".
Chapter 2
The Standard Model
Particle physics phenornena are extremely ive11 descrihctl withiri ttic SM of clemtmt.ary
particles arid t heir fundamentai interactions. Thil SSI proviclrs an &gant t l ieor~t iciil
framework which has successfull:; passeci ver'- prccistl tests. somr of whicti likc sin HL\- =
0.23147(16) have reached the level of 0.1% [9].
2.1 Basic Elements of the Standard Model
The SM describes elementary particles as the point-like constituents of rnatttlr. Ttiiw
are two types of basic building blocks. the matter particles and the interaction particl~s.
T h e matter fields are fermions of spin L/2 and are classified into leptons arid quarks
grouped into three generations. They are surnmarized in table 2.1. The leptons hwr
integral charge and the quarks have fractioriai charge.
The quarks have an additional quantum number called colour. which cornes in three
varieties. The quarks must be confined into the esperimentally observed mat ter part ides.
called hadrons. which are colourless. Thesc colourless composite particles are classified
into baryons, fermions made up of three quarks. and mesons. bosons (integral spin par-
Table 2.1: The fermions of the Stundünl Mode1 and their principd ch~rr~(:tensti(:s.
Generat ion
ticles) made up of one quark and one aiiti-qiiark.
Ali interactions in the SM are mediated by the cschange of mie of several iriterac.r,ioii
particles. which are elementary bosons. These particles arici the forces they nicdiatc arc.
summarized in table 2.2.
Quarks
1
3 -
Interaction
S t rong !
Charge (e)
Elect romagnetic
Table 3.2: The bosons of the Standard Mudel i d thew principal churnçteri.~tics[9!.
Throcighout thzs thesis the convention of rtatuml units (r = h = 1,) is applzed.
up t~
down d
charm c
strange s
Bosoris 1 spin 1 Cliargc ( c i
photon -< i l / O 1 O I l 1 i i 1
The SM is a quantum field theory (QFT) based on the gauge symmety S U 3 ) x
SC7(2) x C(1). This group includes the symmetry g o u p of the strong interaction. SC(3).
This theory is knorvn as Quantum Chromo Dynamics (QCD). The symmetry groiip of
top t
bottom h
Leptons
--
\LUS GC\ 1 1
213
-113
113
-113
Charge ( e )
I 1 i l
213
-113
elect ron-ncir t rino v,
electron e
miion-neutrino Y,
riluon
O
- 1
O
- 1
tau-neutririo Y ,
tau r
O
- 1
the electromagnetic interaction. U(1). is a subgroup of SL:(2) x L:(l). the gauge group
for the electroweak theory of Glashow. Weinberg and Salam[lO. 11. 121. which unifies the
clec tromagnetic and meak sec tors.
2.1.1 Generation of mass
If the symmetry SLT('L) x L'(1) werr unbrokcn. thc I F and Zo woiild br niasslrss wliirti
is not the case. If mus terms were acicled to the SSI lagrnngian. thc tlieory's gaiigil
invariance would be lost. Instead a scalar particle. the Higgs boson (H). is adcled to ttw
t heory. Through a mechanism called spontaneoiis synimetry h a k i n g . the Higgs bos«ii
generates the particle masses while preserving thc ga i ig invariance of the theory.
One assumes the scalar particle H. w1iii:li is clectrically nrwtral. apprars as a doiihlot
of SL-(2) and has Yukawa couplings to both gaugr bosoiis and fcrniioris. Ttic SM potoritial
is chosen such that a t the niinimuni. the H lias a non-zrro tniiss in tlie vaciirrni. Ttic
symmetry SC(?) x ('(1) is spontaneously broken aiid tlie particles acqu i r~ a nias .
Several of the SM particles in tables 2.1 and 2.2 wrrB prrtlict,cd long be fo r~ t t i ~ i r
discovery. and are therefore testaments ta the succ~ss of the SM. Tlirse incltid~ r h ~ gliion
a t Deutsches Elektronen-S'i'nchrotron (DES).). the. II ' and Z bosons at C m t r ~ Eiinqwan
Recherche Niicléaire (CERN) and the top quark and Ï neiitriiio at Ferniilab. Only thr
Higgs boson has not yt been observed.
2.2 Problems with the Standard Mode1
The SltI has been tested thoroughly bu man? particle physics esperinients. )-et. \vit hoil t
the discovery of the Higgs boson it is still incomplete. There are also other theoretical
aspects of the theory which make most believe that it is not the final chnpter of particle
p hysics:
Hierarchy Probtcm
Why is the weak scale so small co~npürcci wit ti the scale of Grand Cnified Thcorics
(GCTS) : - IO-'' -IGIonck:>
Fine-Tuning Problem
Radiative corrections to the Higgs boson masses have quaclriitic divergences. The
corrections ro the Higgs masses are niany orders of rnagnitiitle largrr than thcl Higgs
mass itself. These divergences can bt. renornralizrti away ty carefui adjiistnirrit of
corrections and this rniist be clone to al1 onlcrs in pertiirbatioti tlicory.
Unification of couplings.
In a gauge theory the couplirigs scale witti enttrgy. A ncccssary reqiiireniciit of d l
GL'Ts is that these couplings rtieet at. a single point. .ifter pr~cisc rricastircrnrrits of
the S U ( 3 ) x SL'(2) x C(1) coiiplings. and thr extrapolation of tliesc coiiplirigs to tiigti
energies. unification h a been excludeci by rnorr tliirri eiglit standard dpviationsl K3!.
Gravity
Gravity. dong with the electroniagnetic. and strong forces. make up the foiir
fundamental forces of nature. Gravity is conspiciioiis bu its ;tt)senr:e in the SM.
Chapter 3
P henomenology of Supersymmet ry
One of the problerns alluded to in section 2.2 was the problern of jne-tuning: if t h
fermion and scalar couplings of the theory arp carefiilly üdjustcd. it is possibl~ to caricd
t hc quaclratically divergent cont ri biitiotis to th<% Higgs riiiiss. ;\ synirrietry cari Iw inipostvl
on the theory which results in this cancellatiori to occtir to al1 orders in pcrturbatiori
t heory [NI. It is this symmetry mhich is rcferred to as supersyrrcrnetq.
Supersymmetry (SCSY) relates the masses and cotiplings of particles with diff~rrrit
spins. SCSY is a leading contender for an exterisiori to the SM. It is a gcnernlization of
the space-tirne symmetries of Quant iim Field Thrlory (QFT) ttiat transforni ftwriions irito
bosons and bosons into fermions. SCSY ttierefor~ groiips particles wi th the same mass
and other quantum numbers. but which differ by I l / ? unit of spin. into a superfield.
where Q are fermionic generators wliich satisfy:
{Q. Q} = -Zi,, PlL
[Q. P'] = (Q. Q } = {Q. Q } = O.
where P p is the momentum operator and Aip are the Dirac matrices.
The building of supersymrnetric lagrangians will not be covered here but is tliscussed
estensively in the literature [lj].
SUSY solves the Hieîarchy and fine-tuning problems.
.As previously mentioned tliere exist quadratic divergerices to the Higgs rnass whidi
lead to = O(.\I~,,,,,). Kowever. accorcling to the Fcynnian rules in SCSY.
loop corrections contain both ferniions ancl bosons. wliicti contributt. by a relütiw
negative sign (as illustratcd in figure 3.1) such that:
where .IlsLsi- is a typicai SCSY mas scale. Thi? fine timing problcni tiisappcars i f
th^ m a s scalp for SCSY is of the order of 1 To\*[16]. In otlier wortls. t h fine-t urr~rrg
a h i c h \vas requircd in ordcr to kcep racliativr c»rrrcti«ns iir1dt.r control is rio Io11gc.r
necessaru.
.------- .j Boson * . . . . . -...--.-* -::c.:: ..... .... H o -
Figure 3.1: Radzative corrections to the Hiygs rriass 1n u .s«pt.r.symmetric rnodel.
Unification of coupling constants.
Probably the most favored reason by theorists for the support of a supersymrnetric
Figure 3.2: The evolution of the SLW(3). SL'(2). und C(1) coup1i~~g.s in the (a ) the S.CI
and ( b ) the iChimal Supersymmetric Exteii.si«n to the SM.
theory is that within a SCSI' niociel. iiiiificatioii of the stroiig. t ~ e i i k ilnd rltwro-
niagnetic couplings can be achievcd nt a comnion value of' 10L%Gr[13]. Tliis
perfect unification can be obtained as long as thc SCSY rnasscs are of the ordrr of
1 TeV. Figure 3.2 illustrates this circumstantial evidenc~ for SCSY. The disconti-
nuity a t O(100 Gel*) is due to the dissociation of the unifietl electrowenk force into
electroniagnet ism and the weak riuclear force.
0 SUSY naturally incorporates gravity.
The SCSY algebra cont ains P, generators which arc translations of space-time.
Requiring local gauge invariance under sucli transforriiations leads to the Einstein
theory of gravitation(l3). SCSY naturally is a theory of gravity.
3.1 The Minimal Supersymmetric extension to the
Standard Mode1 (MSSM)
The lISSXI is defined to be the supcrsymnietric extension to the Standard !doclel wliicii
contains the minimal number of new particles and interactions ttiat are consistent with
the standard mode1 gaiige group. The cost of introducing SCSY is the doubling of t h
entire SM particle spectrum. See table 3.1.
Particle spin Sparticlt~ spin
quark (1 L/2 squarks ~ L . R O -
charged lepton 1 1/2 charged slcptoiis I L S R O
neutrino v 1 /2 sn~iitrino fi O
gluon cl 1 gluino + 1/2
a. photon 1 ptiotirio " / 2
neutral higgs h. H. .4 O neutrd higgsiiio f?~,? 112
charged higgs Hz O diarged higgsino H' 112
- I V . Hf mix to Form 2 chargino mass tiigcristates <;. i;
5 . 2 . H:,? mir to forni 4 neutralino n i a s eigenstates &3..1
Table 3.1: Particle content of !CISSICI.
There is one notable feature in table 3.1 which reqiiires clarification. In the SU. one
Higgs doublet is required to give mass to the quarks and leptons. In SCSY. two Higgs
doublets are needed in order to give mass to the up-type and clown-type quarks.
3.1.1 Neutralino Sector
In the neutral fermion sector. the neutral fermionic partners of the and Zo gauge
bosons. f and 2. can mis with the neutral fermionic partners of the Higgs bosons. H P . , . . - to form the mass matris 3.3. Therefore the physical mass states iy. termecl iimtralirios.
are found by diagonalizing this mass niatrix:
.LI1 O -& cos .j si11 f l l l . Jiz sin 3 si11 dtt-
O A& .\IL cos 3 cos Or\- - J I z sin .I cos Hll.
- -LIz cos ,3 sin O r r p A l L cos 3 COS Bu. O -Cl
.CIZ sin 3 sin - .Uz sin .j cos Olt -P O
where is the electroweak mising angle. This rriatris[l7l contains sonie paramst ers of
the MSSX
0 dll and AI2 are the m a s parameters of t he gaiigirios i~ssociated wit ti tlir Le( 1 ),. and
SL'(3) r respect ively.
p is the Higgs miring parameter.
tan 3 is t h ratio of \acuiirri Espectatiori L1iit.s (VEV) of tlith Higgs sciilars.
In order to reduce the nurnbcr of free paranieters i r i the thcory a grarid iinified tlieory
relation is used which relates the gaugino m a s paranieters J I , ancl [13]. - -3
.\II = ; tan' Or\- Jf2.
In general. the mass eigenstates do riot correspond to 5.. 2. K Y and @ but giwn
values For p. JI2 and tan 3. the complicated mixtures of the states is defined. One
c m get an idea of the behaviour of the neutralino masses as a function of the SCSY
parameters in figure 3.3. The masses are defined such that .Clry < JI,? < JI,; < -\1,0. 4
h further discussion of the neutralino m a s sector can be found in (171.
Figure 3.3: Neutralino masses gzven at hxed ( a } JI2 = 100 Ge V ( b ) .Ib = 200 Ge C' and
tan J' = 2 as a functiori of p. <y (solid). f: (dushed) . (3 (dotted) und (rlnsh-dot).
3.1.2 Chargino Sector
-1s in the neutralino sector. th^ physiciil m m statrs of the frrrriioriic partnrrs of the 11 * -
charged bosons. y?-. called charginos. are defined as linear combinations fornicd 11~- ttir
diagonalization of the mass matris:
such t hat
Figure 3.4 shows the mass of the lightest chargino as a function of CL for representatire
choices for the value of .Cl2 and tan 3. By convention the .iT is thc lighter chargino.
Figure 3.4: Chargzno masses given nt fized (a) .\& = LOO Ge V ( 6 ) .\- = '200 Gr C- «nd
tan 3 = 2 as a junction o j p . (dashed) and iFIdot ted) . The (.soiid) iy m a s s is plottrd
as a reference.
The general SCSY superpotential contains terms which violate baryon and lepton nuniber
of the folloaing form:
where i j k are generational indices. L. Q are the left-handed lepton and quark doublet
superfields respectively and the E. D and r are the right-haiicled charged lepton. tlo\vri-
type and u p t y p e quark singlet superfields respwtively. The first two terms of eqiiation
3.9 violate lepton nuniber and the thrid violatcs baryuti riutrilwr. T ~ P SISSM frarntwork
not only considers minimal particle content but also recluircs the tlieory to br mininial
in terms of allowed couplings. The extra couplings giren in 3.9 can b~ aroicletl by im-
posing a strict conservation of R-parity thereby achieving the goal of rnininiizing t h
number of allowed couplings. R-paritu is a riiiiltipliratiw dis(mw syniriietry &fincd as
4 = ( - 1 ) 3 ~ - ~ - 2 s . where B and L are baryori m t i lrptori niinrber rcqmxivcly arid S
is spin. It is defined such that R, = 1 for particles and Rp = -1 for siipersyrnnic~tric
particles (sparticles). r\llo~ving these R, violating couplings ciin leacl to riipicl proton
decay. as illustrateci in figure 3.5 for a non-zwo A' and A". which is exprrinienti~lly vrry
constrained(l81. An ad hoc way to s o h this. is t o iriiposr R, conservation. Anet l i ~ r
Figure 3.5: D e c q of a Proton vra LQD and ('06.
viable[l9] and Less restrictive syrnrnetry is B-parity which ensures baryon number con-
servation (A" = 0) and hence proton stability. It shoulci be noted that in the SM this
problem of lepton and baryon number violating processes does not arise since such inter-
actions are forbidden by the gauge syrnmetries of the moclel.
The consequences of assurning R-paritu conservation are two-fold. Since it is a miil-
tiplicative quantum number the number of SCSY particlm rritist be coriserveci mociiilo
2. In other words. sparticles can only be pair produccd froni S'LI particltxs. Seconclly.
a sparticle will dec- in a cascade iintil the Liglitest Supersyrnrrietric Particlc (LSP) is
produceci. which is necessarily stable. Herice. a niodel wi t li R-parity conscrvat ion \vil1
have a LSP which is stable.
The consequences of R-parity violation (lj!,,) are opposite to the constqiirncps o f R-
parity conservation. Namely tht. LSP will not b~ s td~l i l i i i i d cari (Ir(.- to ortliriary piiît i-
des. .-\lthough the simiiltaneous presencc of lepton and baryon niiniber violatirig rimis
(sec equation 3.9) is severcly corist rai r i c d 1)- txist iiig liiiii ts[ZOj uii t lit1 protori l i f d r i i c l .
G1, searches none t heless offer a ntbw irrid ricli p heriorritiriology.
3.3 Phenomenology of Supersymmetry at HERA
Slodels iri which R-parity is violateci via the SL,Q, D~ term hold tlit. most promisp for
CI state discove- at HERL This is because the leptonic and haclronic flavoiirs in the initi 1
at HERA make it an ideal machine for such a search. The cases in equation 3.9 whcrr
A' # O are especially promising as the- could lead to resonant production of squarks ~ i a
F. - q fusion. with a m a s right iip to the kirlematical liniit.
The search for P, SLSY is the subject of this tliesis. The following sections will
outline the phenomenology of the search performed at HERA froni the production to the
expected final state topologies. -1 similar search has already been performed by H1[2l].
3.3.1 Resonant production of Squarks
One can expand the term L ~ Q , D~ in ternis of its m a t ter fields [El:
(3.10)
where the superscripts c denote the chargr corijiigatc spinors and t h superscripts
deriote the complex conjiigate of the scalar fields. T t i ~ iiitlices L ariti R. for the sciilars.
distinguish the independent supcrpartner fields o f tht. left and right-liaricl~cl fermions.
The interactions outlined in equation 3.10 are depicted in figure 3.6 for A:,,. Among
the 27 possible A:,,, couplings. only the cases with i = 1 are of special intcrest a t HERA
because a P- in the initial state allons for the s-channel production of sqiiarks throiigh
e'-quark fusion as in graplis ( c ) and ( f ) of figiir~ 3.6.
Fiirthcrniorc. HERA is more sensitirr to the X;,, wiiplings t~wuisc t hey allow for thc
fusion of t h e lepton in the initial statcs with a ïal<~ricr quark from the protori.
Positrons and electrons probe different squark coiiplings. In the case of r - scattrring.
e r d + C L -type squarks dorninate the production. .As this thesis is bascd on the data
taken from 1994 to 1997. when HERA \vas operating n i t h a positron heani. the locus
of this thesis is a search for CL- type squarks. Tah l~ 3.2 lists a11 prrtinent resoiiarit
squark production mechanisnis at HERA for positron bcarii. For r z - scattering. e - i ~ -r
- &-type squarks dominate. .\ search in e - scettrririg at HERA awaits the acciiniiilatiori
of more e - data.
The cross section for squark prodiiction at HERA depends ori the square of t h
strength of the coupling If this is sufficiently small then the cross section c m be
approximated by the Narrow W d t h .Approximation (SW.4) [23] for s-channel production
of a scalar resonance:
where q ( x ) z q(x. rn2+ ) is the quark (or snti-quark) rnorrienttirn density in the protori e ( I
and s is the centre of rnass energv of HERA. It is ~ v a l u a t ~ t i at r = in:_ / s and at a '7
virtuality scaie of m:-,. So for esample. o ( e 3 -+ f i L ) depends on the value of A;,, - -
and the d quark momentum density in the proton. For ilR q u a r k production siiriply
substitute the û quark momentum density in the protori. -
For a given coupling strength. the production cross sections for ùL and & are shoan
in figure 3.7. The figure illustrates that CL-type squark production doniinates for e-
scattering. as a direct consequence of the relative magnitude of the quark and anti-quark
Xljc Production process -
111 e ' + i i + d R e - + d + i lL
densities insicle the proton. Due to th^ &op iii cross section with tlio quark riiass. HERA
will be sensitive to lon coiiplings a t loa triasses and liigher couplings are high ninssw.
Two assuniptions are made at tliis point tJo siniplify t h swrrti stratrgy:
Only one of the 4 coupling is non-zero. -4llowirig tiiorr tliari one wiipling to
esist would complicate the phenornenolog.. By isolating one coiipling at a timc and
in the absense of any observed signal. a direct limit can bc derived on the strength
of coupling. Separate analyses[24] have been perfornied at HERA. which set limits
on the product of two couplings but are made in the contest of specific topologies.
more specific than the search tvith which this t hesis is çoncerned.
The gluino. the SCSY partner of the gluon. is much heavier than the squark so
that the decays Q -+ ijq are kinematically not possible. The thrust of this analysis
Figure 3.7: Production cro.s.v sections /or " r.md riR for A' = O. 1 in e p colhsiorrs.
is to cover the SCSY parameter space. Kencc siriw .\II. rvhich is rlitt gaiigi~io riiass
parameter associated ivit h SC'(3) does not enter into t lit! gaugirio n i u s mat riws
3.5 and 3.7 this assumption allows ils to be inseiisitivr to this parameter. T h ~ r r
are additional GCT reIations[l3] (ahere .II:i I= mi) whicli ensure the m u s of t h r
gluino is rnuch heavier tlian the ot her gaiigirios t liiis niakirig t his assuniption al1 t tir.
more valid.
The number of free parameters in SCSY models is inirrierise and surh assuniptions arc
made in an attempt to reduce this nurriber. ivhile wveririg the broadest possible set of
models. The search is in general a topological one. rvith the primary goal to look for
signals which cannot be account for by any SM process. The existence or absense of an-
signal can then be interpreted in terms of the SCSY models discussed in this chapter.
3.4 Squark Decays
The phenomenologv of decay of squarks produced by & is dependent on the strengtli of
the &, coupling. If the coupling streiigth is greater than tliat of the gauge dec-s then tlie
sqiiarks will decay via the same @,, coupling by whicli the? aere rrearecl. Ot herwisr tliey
c m undergo a gauge decay into a rieutraliiio or chargino. which are the only accessiblr
clecays in hISS'LI.
3.4.1 41, Squark Decays
The width for the Pp decay of a scpark is
This aidth[Zl] is wry sinall: for a sqiiark miss o f 200 Co\' i i r i ~ i A;, , = O. 1. it is iipprosi-
mately 40 MeV.
Squark decay. via the 4L, coupling. is s h o w in figure 3.8. The procliiction and <It.ciiy
of squarks in this fashion is very similar to that of leptoquarks. Data selrction and
optimization for such topologies is sirriiltir to t tic leptoquirr k analysis [El. The firial st;ito
is characterized by a positron at high trarisvrrscl Priergy ( E T ) and a jrt.. On ;in weiir by
event basis this final state is indistirigiiishahle froni Siwtral Ciirrent ( S C ) and Chargrtl
Current (CC) deep inelastic scattering (DIS). Nonethelcss. in the arialysis. one can takc
advantage of two physical characteristics of scalar squarks:
The x distribution of a squark produced by nricl decaying via 4 couplhg should be
peaked at r = 5. Hence the search strategy foçiises on searching for a localized
escess above the espected NC and CC r distributions.
A scalar particle in its rest frarne will decay isotropically Hence the cross section
for this decay will be independent of the decay angle O* in the centre of mass frame
Figure 3.5: Squark P, production und rlecay diagrnrns.
of the particle. 0' is related to y. which will br rxplainecl in rliapter 5 . in the
followirig rnanner:
Thus one would expect a Rat IJ distrit~utiori frorti a sqiiark clecaying via &. H o w v r r .
the y distribution for NC DIS. the miljor backgroiiricl for these processes. rlrops likr
I/!/'. allowing for a ver? good signal to t)ackgroiiri<l separation.
3.4.2 R-parity conserving or Gauge Squark Decays
Once produced the squarks can undergo clecays in wliicli R-parity is conserved. Such
decays are the only ones permitted in .\fSSSI niodels n i t h R-parity conservation and as
such the- will decay into a particle and a sparticle. These gauge decys. ivtiich play
an important role when the pl, coupling is small. depend greatly on the nature of the
gauginos.
Choice for the Lightest Supersymrnetric Particle
The identity of the LSP is important to the plienomenology of fi, SCSY. Ttiere are
favorable cosmological constraints wtiich iiriply tliat tlic LSP is tlie liglitest ~icutraliiio
[26]. The lightest neiitralino. as the LSP. in rnodels ivtiicli conserve R-parity is a good
candiclate for dark matter since it is n~cessariiy stable. Horvever. such co~isideratioris are
no longer valid once R-parity is violatd. Nevert heless. the LSP is assiimed to he t hc il;.
There is a sniall portion of phase space in wliicli <r is lighter thüri the lightest
neutralino (see figure 3.9). Since the assurnption for this anaiysis is chat the i: is tiir
LSP. this search is not valid in that sinall portion of phasr space. c w n tliougti chuositig
the 2: as the LSP would riot significantly cliünge tlit! fiiial state topologies in this portion
of phase space. Figure 3.9 shows mtiirh component of thc <y doniinatcs as a fiinctioii of
tlie phase space parameters (.\I.?. I L ) at a fisecl vali i~ of tiiri .j = 2.
The decay ij -+ q i y
The width for the sfermion gaugtb clccay into a quark arid iitwtralinu is givm rqiiatioii
3.15[27].
& Lj-q*': -
rvhere the factor C depends on the 5. 2 and H coniponents of t tir \p. Detailed csprcssions
for C as well as the Fevnman rulcs gowrning siich '\1SSSI rtlactions c:;iri tw foiincl in [l7].
This d e c q becomes more significant as the differericts iri rriass betneen the squark and
neiitralino increases.
The decayq -+ q' y
Charginos are mixtures of the charged wino ( l i - ~ ) and charged higgsino ( H Z ) States. It is
important to make a distinct.ion at this point betwen the raciicaliy different dynamics of
Figure 3.9: Regzons in the SUSY phuse . s p ( ~ c ~ ( J I 2 . 11) ct~npspondzrlg to u charged LSP
tjqlf= couplings dcpending on whether the dccqping sqiiark is thtl SLSY partnrr of a lcft
or right-handed squark. Take the right-tiarided q u a r k for rxample. T h e weak interaction
only couples to left-handed fermions. so the II'' does iiot couple to right-handcd fermions.
Sirnilarly in SCSI-. one cannot cou pl^ a wino to a right-limdcd squark. Iri rtic casr of
a chargecl higgsino coupling to a rigtit-liaridd sqiiark. t h v ~ i ~ s !vil1 b~ proportiorial to
the m a s of the quark (q') . The width of ttir tlec:q GR -t q' H= is proportional to mi,.
Should the quark be in the first or second generations tliis width is negligible. Thereforr
whether the f' is 1iW= or fiz dominated. it is urilikely to have a significant probability
of occurring.
For a left-handed squark (the dominant productioii in t his analysis) t lie coupling to
a aino is not forbidden. -4 higgsino dominated ,ii will stiffer froni suppressions similar
to those for ijR decay. The qL decay widr h is given by [?Tl:
1 r,+R,tS = - ( c~)~~- \ I+ Qîï
(3.16)
where the details of C are give in [ l T ] . The ciL decay niode is clominated by its tlecay
into n chargino as soon as it is kinernatically possible. This is clenionstrated in figiirr
3.10 where the dominant gauge clecay niode is reprcwiitetl for a 150 Ge\. f i L sqiiark i n
t h e phase space plane (&. p ) for tari 3 = 2 .
Figure 3.10: Dominant gauge decuy mode for u 150 Ck\' .sqirurh- ln the phnsr .spirr:P
plane ( J I 2 , p ) for tan 3 = 1[28].
3.5 Neutralino and Chargino Decays
It is necessary to study the decays of the gauginos in order to determine the possible
final state topologies when the &, produced squark has a gauge decay.
3.5.1 Neutralino Decays
In order to study the decays of the neutralinos it is cssential to discuss the dynamics of
the LSP. In models of the SLSSSI in whicli R-parity is conserveci. the LSP is stable. Hencr
any final state topologv for such rnodels consists of a characteristic missing transverse
mornentum ( f i ) coming from the LSP which escapes iiridetected. On the contrary. in
models with j?,,. the LSP is in general not stable anci ni- circ:ay irito a ferniion and a.
virtual sferrnion which subsequently d e c q s through an &, coupli~ig. Such tlccay chairis
are represented in figure 3.1 1. When the dcc-s irito ii cfiiirgecl lepton it can rleciiy
Figure 3.11: Representatiue diagrurris jrom the pl, d e c q o j the for a non-zero
For n = 1 these diagrarns represent the Jecays of the LSP and the finrd .st«gc of rrny
cascade. The charge conjugate diagrarns are not draun.
with equal probability to e' or e-. The wrong sign lepton (e'p + e - -+ multijets) is
the most unambiguous and background free signature for the production of a squark via
R*
The branching ratios for which f y decay is dominant depends on the nature of the
f y . i.e.. which cornponent dominates the mixture: the 5. z or f io . Recall figurr 3.9.
The branching ratio for the + e x j e t s is mauinium wheii the iy is dominatecl by irs 5
componcnt and hence the coupling strength is proportional to the charge of the lepton.
If we consider the ciecay of y!!, nhere n > 1. they can also dec- directly via $ as
shown in figure 3.11. In addition there can be radiative decays such that iy,+ 7 or Z
,?y. as illustrated in figure 3.12. The in,, are allowed to iindergo a gauge ~lecay wlicrc~
Figure 3.12: Guuqe Decags of the f: for n > O. ni < n .
R-parity is conserved. The py will decay as described above.
3.5.2 C hargino Decays
Chargino decays become very important once they are kinernatically possible as already
illustrated in figure 3.10 and are the dominant decay mode in most of the considered
phase space. As in the case of in, ,. the Xg's can decay via P, A',,, couplings in a sirnilar
fashion to the decays shown in figure 3.11 or undergo gauge ciecays as in th<? esamples
shown in figure 3.13. The # subsequently decays in the assumption of 6% as clescribecl
Figure 3.13: Grrugr: decug rrrodes for the i,'.
in the previous section resul t ing in ario t her cascade dec-
There is an enormous nuniber of possible (lecay modes and an rffort nns niade to
consider the most significant modes in order to make the most comprehcnsivc srarch
possible. In that respect. cascade clecays involving the and were not considered.
That is to s q that the assumptiori is made that in tliis analysis the- are not ohseneci.
The? can occur but the efficiency for their detection is effectively zero. This is reasonnble
because q' + q~: , , and ij + q'?; channels are rarely or eren never a dominant squark
decay mode as seen in figure 3.10.
3.6 Possible Final State Topologies
The different decay modes discussed above can be classifiecl into separatr topologies
depending on their final states. The topologies considered in this search are siimmarized
in table 3.3. Table 3.4 siimmarizes other distinct topologies which were not iriclucled iri
this search. Wi th positrons in the initial state the &-type sqiiark prodiiction clorniriatcs. -
Hencc the final state frorn dR -+ vq. consisting of one jet and high PI' is not consiticrd
Table 3.3 shows al1 the final s t a t ~ s considerecl in t his search ahich di& in the lepton
identity and the riumber of quarks. Esperimentally no distinction is made bctween t\vo
or more reconstructed jets in the final state. Hence. one can sec ttiat althoiigh thcrc
are seven quoted firial states. there are only four recoristriictetl chaririels. -411 t h fina1
states are considered in terms of rfficicncy of c l c t ~ t ion h i t srperatr opt irtiizat ions for r Iip
separate rniil t ijpt topologies arc iior rriacle. This is disciisstd flirt hor in c h a p t ~ r 7 rvliilrr
t lie analysis met hods are explaineci. The rsaniplc deciiy proc-rsst3s err iilso reprcwiit iit iw
and riot cxhaust ive.
3.7 Summary
SUS\* is a compelling theory for an cstension to the SM. hiit as y t lias no esperimental
support. The phenomenology of & SLSY at HER.4 has been outlincd ancl the assiinip-
tions made in this particular search were described. The niain points are sumniarizd
liere for convenience.
The SCSY process under stiidy is the rcsonant production of a squark througli
the fusion of a positron incident on a quark froni the proton. This niode violatcs
R-parity via an ~ ~ & , 4 ~ operator.
In order to study t.his particular SCSY mode1 the following assumptions were made:
- The LSP is the lightest neutralino.
- The gluinos are much heavier than the squarks producecl so the decq- <j -i jq
is not kinematically allowed.
- Only one of the X i l h couplings in equation 3.10 is doriiiriarit.
- Although each A l j , coupling allows for the procliictioii of two iiiiiqiie sqiiarks.
(iL and C R . the productiori is doniinated hy CL-type sqiiarks wlicri HERA is
operating with a positron bcarn (see figure 3 .7) .
The squark may dec- via & or gauge clec-
- hfter making the assiiniption in equatiori 3.6. tlic ncutralino ancl diargino
sectors are defiried by tlirec SCSY parameters: .\-. thil n i a s trrm for the
SLV(2) gaugino. p. the Higgs misirig parairieter and r;iri .A t h c i ratio of V E \ 3
of the Higgs scalars.
- The mode1 consideretl is an iinconstrained h[SS'\I nitti &. Tlic sfcrniim
masses are free pararneters and are taken to be mass d~gerieratc.
- By allowing for &. thc LSP is iiot stable and cxn clccay as illiistrat~tl iri figiire
3.11.
The final state topologies consiclerecl arc listed in table 3.3. -411 ciwacle tlecays in-
volving & and <: were taken into account. Four distinct reconstriictetl channels are
searched for:
e' + 1 jet
e' + multiple jets
e - + multiple jets
PT+ multiple jets
Decay Process -. 1 Reconstructed 1 Fiiial ~ t a t c j
Channel 1
High & e' + 1 jet P 3 1
1 High 4. e+
A' 9 e-qtqt' 1 + niuitijcts /
High &- 1 I "qqq I
+ niiiltijcts i i
Table 3.3: Squark decay channels in 4% SGrSY which were considered. c/assified hv their
final states und their reconstructed topologies. This list 1s onlv r~presen tü t i ve and nol
exhuustive.
/ Nature of LS? f l 1 Demy proccss Topology
Nissing PT + 1 jrr
High e=
- High Pr P * or p - 4 &-+ triiiltijrts
Table 3.4: Squurk Decay Chanrieh in P, SUS Y classified bv distinct topologie.s.[27/ These
channels are not considered in the seerch urld U S in tuble 3.3 this list 1.9 .sot exhav.sti.ue.
Chapter 4
The Experimental Setup
Particle accelerators and colliders have proviciecl t tie experimental cvideriw for th<. basis
of the SM. They will also be the kcy to providing hints of nctv physics I~eyoncl thr
ShI. or perhaps Pven stimulate the development of ritlw tticoritts. Thp Hadron Elektroti
Ring .inlage (HERA) Ilas alreaciy p laod aiid c:ontiniicts t.o (wnrrit)iitc~ an iniportant roltl
in this aspect. In ttie followiiig stxtioii a brief ovrrview o f thcl rriachint' itsdf ;lri(l t l iv
ZECS detector is prescntcd. ZEUS is one of ttie tuo large drtwtors at UER.-\ and
is a collaboration of about 430 physicists represcntiiig 12 countries. A more dctailed
description of the ZEUS detector can be Founcl in the 1993 Stattis Rrport[29].
4.1 HERA: Hadron Elektron Ring Adage
HERA is the world's first and only lepton proton collider and is locatrti at the Deutscties
Elektronen-Skrnchrotron (DESY) laboratory in Hamburg, Germa-. The HERA acceler-
ator complex (figure -4.1) is designed to accelerate electrons to 30 Ge\- and protons to 820
GeC'. yielding a center of mass energy of 311 GeV. The ring has a circuniference of 6.3 km
and runs 10-30 m below ground level. The design energy of the electron beam is limited
HERA Parameters
Circum ference (m)
Encrgy (Ge\')
Slagnetic Field (T)
Luminosity (cm-%- l )
Number of colliding bunches
Bunch crossing time (ns)
Ciment (ni;\)
Horizontal bearri sizct
ffr (mm)
Vertical berzni size
ov (mm)
Longitudinal beam size
0: (cm)
Energy loss per turn (Ge\-)
Injection Energv (Ge\*)
Injection time (min)
Design Valiies 1 1997 Values
Table 4.1: HER.4 deslgn and 1997 running parameters.
I electron 1 proton electron proton
Figure 4.1: Layout o j the HERA accelerator cornplex ( ~ r r d irljectiorr sg.stein.
by the radio freqiiency power required to rompensatcl for rnergy llost diw ru syic-lirorrori
radiation. while the design energy of the proton beam is liniitccl hy t l i e 4.65 T iiiitxi~tiiirti
magnetic field of the bending ciipoles [SOI. Tlie counter-circiilatiilg t4wtron and proron
beams are contained in tno evacuated storage rings aritl collicie in the H 1 m c i ZECS
experiments. which are situatecl in the north (NORD) and south (SCDI ~sperimental
halls. respectively. The parameters used by HERA in 1997 are listtd in t a b l ~ 4.1.
Figure 4.2 shows the accumulated luminosity of HER-A since 1992. The improving
performance can be seen in the iricreasingly steep slopc y a r by year.
HERA luminosity 1992 - 2000
1 0 0 150 200
Days of running
Figure 1.2: The integrated lurnino.sit?j recorded bg HERA /or 1992-&/O0.
4.2 The ZEUS Detector
The ZELS1 detector at HERA is a general purpose detector and has been in operation
since 1992. The detector has a large forward-back~vard asyninietr- to accomriiodate the
boost of the centre-of-rnass in the direction of the proton beam. ahich is caused by the
Figure 4.3: Cutuurmy uieui oJ the ZEUS detector.
large asymmetry in the encrgies of the elcctron/positron and proton beanis. The right-
handed ZELS coordinate system is aligned sucli that the i asis points in the proton
beam direction with the origin at the nominal interaction poirit. ariti th .r asis poinririg
towarci the centre of the HERA ring. The arigles O and m are rncasiired relative to the :
and r axes respectively. The ZEUS detector is made iip of many coniporieiits. of wliich
only those of particular relevance to this analysis will be discussed. and is shown in figure
-4.3.
4.2.1 The Central Tracking Detector
The central tracking detector (CTD) provides a measurement of the direction and mo-
mentum of charged particles with high precision [31. 32. 331. It is a cylindrical drift
chamber made up of 72 concentric Iayers divided into nine sup~rlayers. The active vol-
ume of the CTD has a length of 205 cm. an inner radius of 18.2 cm and an oiiter radius
of 79.4 cm. The CTD operates in a 1.43 T solenoidal magnetic field. Tracks are recon-
structed in the angular range 15" < 0 < 164" and there is full coverage in azimiithal
angle 0.
A gas mixture of Ar/CO?/ethane and ethanol is iised to till the tracking volume.
Argon. which is relatively inespensive requircs orily ii low iritcrisity elcrtric field for
avalanche formation. forrns the niain component of the trackirig ctiambcr giis. Gascs
consisting of heavy organic niolecules. siich as carbon-dioside and d i a n e . are known as
'quenchers' [34]. Such 'quencher' gases have iiiariy degrces of Erccdoiti aritl caii efficient ly
absorb energy from the avalanche in ortler to stop it. The carbon-cliosidc also lessens
the flarnmability of the mixture [Xi. The quenchiiig proccss rlis<:iissrcl abow nia' o i ~ a -
sionally lead to polymcrization in whicli liquiil or solid polynwrs drposit or1 t h (.liariihrr
wires. ahich scriously affects the operation of tht: drift ctianihw. -4 non-polyi~ierizing
agent like ethanol is added to retliice t his ~ffect . Charged particlrs trawling t hroiigh tlir
CTD ionize g s atoms. The crnancipat~d clectrons tlien t ravcl azirriiithally to t h ariocltl
sense wires where they avalanche ancl musr a measilrable electric piilsc.
The sense and field wires are divideti into octants. onc of which is showii in figure
4.1. The heavier dots indicate sense wires while the lighter dots represent field wires.
Field wires are held at varying potentials to create a uniform electric field. In cvery ottier
superlayer the wires are strung parallel to the beam a i s . while the wirc.s in the otticr
superlayers are angled approximately fiw degrees from the beani a i s in order to nieastire
the 2 position of a track via a stereo effect. In total. the CTD consists of 4608 sense
wires and 24192 field wires. Superlayers 1. 3 and 5 are instrunientetl with a r-by-timing
system for trigger purposes and have z resolution of approximately 4 mi. The resolution
of the CTD in r - o is about 230 pm. resulting in a typical event-by-event interaction
F i e 4.4: One octant of the CTD showing the field «rd sensr iuzrrs. T h stttwo (inrglr
for the stereo layers is also displuyed.
vertex resoliition of 0.4 <:ni in 2 and 0.1 cni in the trariswrsc plaiic.. Alttioiigh tiic A y -
timing system has a poorer rcsolution. it provitles a fast sigrial to b~ iisc~l by r tir) t.riggc'r
system to reject bcarn relateci background. The transverse rrioniibritiini ( p , ) resoliition for
full-length tracks can be parametcrized as a(pt ) / p l = 0.005Spt 8 0.0065 0.0014/pt [36].
with pt in Gd'.
4.2.2 The Uranium Calorimeter
Calorimeters are devices which measure the total energy deposited by a particle or grotip
of particles. The absorbed energy in the calorimeter is converted into a meastirable signal
which is proportional to the energy of the incident particle. One class of calorimeters are
sampling calorimeters which are cornposed of layers of active niaterial interleawd with
layers of an absorber. The incident particles interact witli the materials in the absorber
Iayers. producing secondary particles which in turn interact and leacl to the development
of a particle shower. The active iayers 'sample' or nieasure the energy flow as a hnctiori
of dept h. The characteristic lengtli of electromagnetic sliowers is srnaller t han tliat i,f
hadronic showers of comparable eriergu. As a result. electrornagnetic calorimeters are
found in front of t heir hadronic counterparts.
Electrornagnetic showers consist of electrons. positrons and photons. At enrrgit3s
above 1 Ge\' the following processes doniinate an electroniagrietic: siiowr:
Bremsstrahlung: The radiation of photons. froni elcctrons and positrons. iindcr thil
influence of the field surrouriding heavy niiclei. The ciiiwgy loss is proportioriiil t.o
2'. where Z is the atomic number of the material travcrsccl.
Pair production: Photons witti enough energy can produce an ~ l w t r o n and positron
pair under the infliieiice of the field of niiclei. Thta rross section is proportional to
z"
At l o w r energies:
Ionization: Electrons and positrons losc energy by ionizing the traversecl mcdiiim.
The energv loss is proportional to Z log Z.
Photo-electric effect: When the photon is absort~ed by ari atomic electron. the atoni
cari be ionized. The cross section for tliis proccss is proportional r.o Z4 - 2'.
dominating üt lower photon energies.
The scale for electromagnetic showers (:an be expressed in terms of the radiation lenyt 11.
.Yo. and is material dependent. -Yo is defined as the energv loss of electrons tlirougli
bremsstrahlung a t energies above 1 GeV:
where x is the thickness of the material and E is the energy of the incident electron.
Incoming hadrons not only undergo electrornagnet ic. but also nuclear interactions.
The tiadronic shower is niore complicated due to the enorrnous variety of processes. Tht.
secondaries. in hadronic showers. are riiainly createrl in inelastic collisions witti niiclci iii
the absorber material. The folloning processes contribute to the drwlopment of liatironic
showers:
production of chargcd hadrons that lose energy by ionizatiori iiritil a riew strong
interaction occurs.
e production of neutral hadrons that only undergo strong interactions.
0 production of neutrinos w hich escape the calorirnctcr uiiclctcctr~cl.
0 production of particlcs nhich shocver electroiiiagiietically.
0 production of escited nucleoris. tv hich releas~ low ericrgy p tiotons or nticlwns o r
which undergo fission.
The hadronic shower starts to die out. wheii the tmirgics of the shower parti&s him~rnr
so srnall. that they are completely ahsorbecl. Thr scalr for Iiarlroriic showcrs cran tw
erpressed in ternis of the nuclear interaction length. or the n iwn frer path betwrii
hadronic interactions. It is defined by:
shere -4 is the m a s number of the absorber. p is the specific density. .V.., is Avogadro's
number and o, is the inelastic cross section [37].
The ZEUS calorimeter [35. 39. -101 is a sampling calorirneter aliich consists of al-
ternating layers of 3.3 mm thick plates of steel-clad depleted uraniui~i as the absorber
and 2.6 mm thick plates of plastic scintillator as the active material. The ratio of the
BCAL
Figure 4.5: The ZEUS calorimeter (9-2 projection).
thickness of the uranium to the scintillator is chosen tu achirvi. cwniptwsation. i x . t h .
responsr to hadronic and electromagnctic particles is eqlial ( e l h = 1 .O0 zk 0.0;3). ;111d
to give a good energy resolution for lia(1rotis and jets. The rvergy rt.soltit,ion of thc
calorinieter is a ( E ) / E = 0 . 3 s / J m ~ for hadrons and n ( E ) / E = o.~s/JFGG for
electrons/positrons. as tneasured in a test beatn. The calorinicter also provides an accu-
rate timing nieasurement with a resoliition of ~ ( t ) = l.5/,/= 3 0.5 ns in n s i~igk
ce11 ivith energv abovr 3 Ge\'.
The calorimeter is divided into parts. ai th each part being longittidinally segmcntctl
into an electromagnetic (EMC) section and one or two hadronic (HAC) sections (figure
4 . 5 ) . The fonvard calorimeter (FCAL) covers 2.2" < 9 < 39.9" and contains riva hadronic
sections. The barre1 calorimeter (BCAL) covers 36.7" < B < 129.1" and also has tno
HhC sections. The rear calorimeter (RCAL) covers 1'28.1" < H < 176.9' aiid has only
one H X section. since hadrons individually have low energy in the RCAL due to the
forward boost. The ESIC sections are 25 radiation lengths thick and contain most of the
energy of an electromagnetic shower. The total depth of the calorirneter ranges froni 7
nuclear interaction lengths in FCAL to 4 interaction lengths in RCAL. The calorinietcr
is hernietic escept for a 20 x 20 cm hole in FCAL and a 20 x S cm hole in RCAL to
accommodate the HERA beani-pipe.
The scintillator light. from each calorimeter cell. is rcad out by two wavelcngth shiftcr
(R'LS) light guides on opposite sides of the cc11 attactiecl to ptiotomultiplier tiihcs (PSIT).
providing redundant left/right readout . Therc are -- 10000 P SlTs iri the c;ilorinict(x
The natural radioactivity of the deplered uranium provicles a stable refcrcnce signal
which is used for calibration of the calorimeter. Csing this signal. rtie encrgy calibration
of the calorinieter is precise to 1%.
4.2.3 Other Components
Other subdetectors of the ZEUS detector illustrateci in figure 4.3 are bricfi! drscr ihd
here for completeness but were not central to the analysis which is drscrilwri in this
t hesis. An instrurnented-iron backing calorimeter[-l il (BAC) rritwsurcs cnmgy leakagtl
from the CAL. The muon chambers in the forward['l9] (F'rICOS). rear (RMCO'i) and
barrel[-E] (BhIUO.\:) regions are used to masure muons in ep physics rverits iind to i t h -
tif? and remove background events induced by cosmic-ray or beani-halo muons. The rear
( RTD) and forward ( FDT) t racking detectors are planar drift chanibers w hich provide
supplement tracking information which can be combined with the CTD to reconstruct
particle tracks in the regions outside of the full-length track CTD acceptance. The FDET
is a composed of three planar drift chambers. interleaved with two transition radiation
detectors (TRD).
Figure 4.6: The Lagout of the ZEUS lurninosit~ rrtonrtor as rue11 u s sorrir of the HER.4
quadrupole (Q) and dipole ( B ) nragnets.
4.2.4 The Luminosity Monitor
In order to nieasurc a cross section. the delivcred luminosity mtist br detcr i i i ind Thr
liiminosity for ZEUS is determined by rneasuritig the rate of harci brerrisstrahliing photons
from the Bethe-Heitler process ep + elyd [-431. an elcctrornagri~tic proccss whosr cross
section is precisely known to an nccuracv of 0.5%. Thus. a precise rneasiiremcnt of t h
photon rate allows for precise tleterminatioii of the e p liiminosity at HERA.
Photons from the Bethe-Heitler process a i t h 0, < 0.5 nirnd exit tlw heani-pipe
clownstream of a large vertical bend in the proton beam line. The photons iirr tiien
detected at 2 = -107 rn by a lead/scintillator sampling calorinieter (LCSIIG)[-H] as
shown in figure 4.6. A carbonflead filtered is installed in front of the LCSIIG detector
to shield the detector from synchrotron radiation. The LCSIIG drtector has a resoliition
of cr(E) = 0 . 2 3 / , / ~ ( ~ e V ) . The luminosity is deterrnined froni the cross section. the
counting rate of photons and the photon acceptance.
A small electromagnetic lead/scintillator calorimeter (LCSIIE) at 2 = -33 m de-
tects etectrons between 7 and 20 Ge\' produceci at O, < .3 rnrad with respect to the
electron/positron beam direction. Tiiese electrons are cleflected niore by the HERA
magnet system than electrons/positrons at the beam energy and leaïe the bearn-pipe at
2 = -27 m and impact the LCSIIE detcctor. The LC'SIIE detector has a resolution of
a ( E ) = 0 . 1 8 / \ I ~ ( ~ e ~ ) . Coincidences of botti the electron/positron and photon froni
the Bethe-Heitier process in tlie LCMIE and LCSIIG detectors allow the LCSLIC: ta 1 1 ~
calibrated.
4.3 Trigger and Data Acquisition System
4.3.1 The First and Second Level Trigger
The HERA heam bunciies cross once every 96 ris givirig a brani m ~ s s i n g ra t t o f 10.4
MHz. In cont ra t . interesting physics evcnts occiir at ;i rat<. of O(100) Hz eiiti t tic.
output of recorded cvents is limited to a rate of O(10) He. The tiigli background ra t rs
predominately consist of beani-gay events. which are largely causecl hy t hr proton bt!ani
interacting with the residiial gas in the beüni pipe and t hc walls of t hr bcarri pipr upst rrarri
of ZEUS. A sophisticated trigger decision is necessa? to filtcr t h r interesting ~p rvonts
out of the large background rate. Sucli a trigger decisiori cannot be macle tvithin the
96 ns bettveen bunch crossings. Therefore. in orcler to niinimize dead tinie. the ZEUS
trigger uses a three Ievel. pipelined trigger system as s h o w in figure 4.7. Even thoiigh
it reqiiires 0.3 s for al1 three levels of the trigger to analyze au event. the trigger is able
to annlyze every beam crossing due to the pipelining. Additionall. since each evcnt
contains roughly 100 KByte of information. only a small rate of events can be written
to disk/tape and the trigger selects a nianageable rate of less tlian 10 Hz froni the 10.4
MHz beam crossing rate.
ZEUS detector cornponents +
data rate 10 MHz L.
front end 4
E 2 L -
4.4 p e c pipeline P r . I
readout and local E T - - - - --- - -
Equiprnent. Global n T - output rate - 1000 H GFLT j Ct~mputef
Equipment = - Computer .L
output rate - 100 Hz i -' t -l-
I
Event Builder I l y [distribution Equipment I
collecting subevents : - via TP networki ; Cnmputer
- - - - 20 Mbytedsec ,
TLT computer farm consisting of 17 Silicon Graphics output rate 5-10 Hz
dala tnnsfer to main -& m a s stonge I
2 k - Data Quality' - SIonitonng
Figure -1.7: Schematic diagram of the ZEUS tngger and data ucquisition systeni.
Each ZELÇ subcomponent has its own first level trigger (FLT) electronics. the main
FLT subcomponents being the calorimeter and CTD. These electronics have availablc to
them very coarsely measured quantities which they analyze using pipelincd logic and afttrr
approximately 25 bunch crossings they report their results to the global first level trigger
(GFLT). The GFLT issues a global trigger decision büsed on varioiis logical conibinations
of the information from the subcornponent FLTs. This final tlerisiori orc:iirs 4.4 p s huricli
crossings after the original event occiirred. T h e is a srnall deati tinii. from stoppirig tlir
pipelines at the first lcvel to read out the front crid data froni thc coniponents after the
a first level trigger.
Data From events accepted by the GFLT are harided to each sribcomponent's sccoricl
levd trigger (SLT) mhich analyzes the data usirig its own transpiitws and electroriics. At
the second level. the data arc rnostly digitized and arc triore preciscx thari i it t h t b FLT.
The global second level trigger (GSLT) uses transpiitcrs to proctlss t h information from
the subcornponent SLTs and is able to use more complcs algoritlirris wliich recltiirr niorp
processing time than the GFLT wlien deciding t.o accept or reject an ewnt .
4.3.2 The Third Level Trigger
Events accepted by the GSLT are handecl to the everit builder (EYB) wtiich comt~ines
and formats the data before sending it to the third level trigger (TLT). Thc TLT is the
first trigger level which has access to the cornplete data flow From d l components of the
detector and is able to use sophisticated algorithms to pick out specific physics channds
from the data. The TLT is the first trigger level to have access to the coniplete raw et'erit
data and hence the global quantity of an event can be esploited. In principle any offline
selection can be performed online. limited only by the CPC power. The TLT is desigried
to reduce an EVB rate of - 100 Hz to 3 - 10 Hz. and this is accomplished by running
Event Builder 8 (Tra asputer Net w or k)
1 I
I
f Network Switch
ta Arehjving m. Two Tmnqvrer Board RAD; Radnont €ME-R>DI.l Baùrd
Figure 4.8: The TLT harrlwtrre desryn.
an optimized version of the reconstruction software. In particular. a t rack reconst riiction
package is run on the information from the CTD [-E]. As of the start of 1997. t . 1 ~ f u l l
tracking reconstruction code. the sarne one used offliiie in reprocessirig. was used onlint)
at the TLT.
The Hardware Design
The TLT hardware design is clepicted in figure 4.8. On a positive decision of t h e SLT.
the EVB gathers al1 the component data and writes the whole event into \'ME rncmory
Two memories are read out over V'rIEbus by one PME-FDDI-1 board (manufactured by
Radstone Technology PLC) on which WindRiver's operating system VsWorks is running.
On request. the PME-FDDI-1 board sends events via fiber optic loop t o a terget CSIS
6 m Evm t Bui idcr
I
Figure 4.9: The TL T su ftwum deszgn.
workstation. The farm of workstatioris is coniposed of 14 Silicon Graphies Challengt. S
CNIX stations. cach containing an R4-100 SIIPS Technologies processor. aritl 3 Silicon
Graphics Origin 200 U N I S workstations. each containirig diid R 10000 SIIPS processors.
The workstations are al1 connected via an FDDI loop and connectetl to a netrvork siritch.
hence onl?. outgoing packages to the archiving workstat ion cross the switch.
The Software Design
The TLT processes are divided into control. transport and monitor Iayers which oper-
ate independently from one another. apart from the interprocess communication. Al1
processes in the system are event driven. Furthcrmore. the proccsses are ordered in n
hierarchy as s h o w in figure 4.9. The TLT Run Coritrol is the top level which interfaces
ni th the ZECS Central Run Control (CRC) and Iiandles al1 state transitioris of the TLT.
The states are. IDLE. REA D Y ancl ACTIVE. The nest Ievcl i r i thc liierarchy are ttir
Brandi Controllers which in turn are in charge of the Analyzers.
1 1 1 processes of the control process layer deal with the state transitions. -411 control
processes wait for niessages from the appropriate processes in the hierarchy and arc
able to synchronize through an eschange of state information. In tiiis mu. a restartwl
process can join the systern in üny state or state transition. The -4nalyzt.-Controllcr is
the parent process of the Input and Output tasks and of the Pbysics .-\nalyzcr. Thrrr is
one Analyze-Controller and Physics .Analyzcr per processor.
The transport 1-er handles the data frotri frorri the EVB to thi. workstations. Th.
Evmt-Distributer. running on the PME-FDDI-L board. sc~itls rlvrltits oti rrqiicst to rl i (1
Input processes. The events are ivrittcn to o. niiilti-pwnt-buffw oii tlii. assigrid work-
station from where. iipon a positive clecision of the physics filtcrs. tliry arp sent to t h r
archiver.
The only process in the filter layer is the Phvsics Analyzer. \YiiiIr twirig analyzcd.
an event remains in the multi-ewnt-buff'er. and the results of t tit. physics dccision are
appended only if the event is accepted. For monitoring purposes. the filter job kceps track
of various statistical data during a run. The information is written to sbared mernoru.
ahere it can be accessed by the monitoring layer processes. The nionitor processes have
no impact on any of the other processes in the system but offer the only method of
monitoring the performance and qualit- of the data.
Figure 4.10: flou.^ c h r t outlining the TLT tngger decision.
The TLT Trigger Decision
The TLT trigger decisiori is made iri three stages. shown in the Hocv cliart in figure 4.10.
By the time the data has reached the TLT. niost of tlie non-rp backgrounds have twen
rejected. Nonet heless. a final strict rejection criteria agninst t his non-ep backgroiirid is
perforrned in the first 2 stages of the TLT decision. At the TLT ttiere are 4 classcs
of vetoes applied. ahich rely on the calorimeter and track reconstructio~i and muon
identification. The vetoes were designed to reject "sparks". beam-gas and cosrnic and
halo muon events. and miist provide fast rejection of background ahile niaintaining a
high efficiency for physics events.
-4 spark occurs when a calorimeter PSIT housing a t high voltage discharges to groiiriti.
The rate is low but significant for - 10000 PSITs. In general. a spark occiirs in only one
PSIT in a given ce11 and may bc identified by a large lelt-right iisyriinietry in the wlI
energy. An event is classified as a spark if it satisfies:
Erents are rejected if they contain a single spark ancl less than 3 GeV elsewliere in t h
calorimeter.
In order to reniove residual beani-gas events. the calorimctcr timing information is
iised [46]. .As illustrated in figure 4.11. particlcs coniing frotri e p interactions at the
interaction point (IP) arrive almost, simultaneously at the idorimeter k i t t = O.
The rejection algorithm calculates a weigtitetl average tiirit. for the FCAL. RCAL and
global regions (the entire CAL) using PSlTs with at l e s t 200 MeV. Thr TLT reacls in a
list of known bad channels From the calorimeter. Only those wlls with two good PSITs
with an asyrnrnetry 1 ;% 1 < 0.2 are inrluckd in thc tinie nieasiirpriient. Tlitl error on
the tinie of each PUT. 0,. is parameterizcd as a furirtion of the PUT tirirrgy. E,:
The time average for any region c m is given by:
with error
The energ-y thresholds for the calulations given in equations 4.4. 4.5. and 4.6 to be made.
are 1 GeV in RCAL and the global calculations and 2 GeC' for FCAL. An event is vetoed
e-p collision
beamgas interaction
Figure 4.1 1 : Time meusurernents for ey und berirn-gas ~nterncttons.
(4 (11)
Figure -4.12: Tinie distributions of RCA L t2rne.s oersvs the FC.4 L nun us RC.4 L t m t . i«
bejore und (b / after the TLT trigger decwmn.
if there is sufficient energ. in a region and if one of the following cotiditioris arc satisficci:
I t ~ c . 4 ~ 1 > ma@ M. 30, ,,,, ) ( 4.7)
> niau(8 ns. ht,,., , )
Figure 4.12 shows how efficient the timing cuts are iri scparating the major beanigas
background from the ep candidates.
If an event survives the spark and beam-gas vetoes. a muon rejection algorithni
MUTRIG[~~] is applied a t the TLT. in order to reject cosrnic muons or proton beam re-
lated halo muons which can traverse the detector. Esamples of such events are given in
figure 4.13. The rate of cosmic and halo muons is substantially lower than the beam-
(a) Cosmic ( h ) Beani Halo
Figure 4.13: Examples of cosrnic and halo muons in the ZEUS detector.
gas background but nevertheless can be of O(10) Hz. To be sure that no iriterrsting
events of possible n e w physics are being lost. vetocd (?vents flaggd as cosmic or halo
mitons are passed throtigh two esotic physics triggcrs. ES011 ü ~ i d E S 0 1 2 iiiiikr i i s ~ of
the GLOMU[-LS] package. a more sophist icated rriiion iderit itirat iori roiit irio r o rrrowr t liostb
events considerecl tu be cosmic or liülo miions with oril- a marginal probahiliry
Finally, events are processed through the filters and arp kept only if one of tlit! physics
triggers fires. A provision is niade. at al1 levels of the da ta acquisitiori (DAQ) chairi. t,o
accept a certain rate of events which are simply prrssd tliroiigh thr trigger rpgartllrss o f
its decision. These are useful for rate studies for netv triggcrs. upgracling ttsisting m r s .
helping to determine trigger efficiencies and evaluating the clara qiiality.
The Physics Filters
The current physics software is composed of over 5OOOO lines of predoniinantly fortran
and some C code. There are 125 filter dots classifieci into five major groups relating the
class of phpics events selected. The groups are soft photoproduction (SPP). liard photo-
production (HPP). deep inelastic scattering (DIS). heavy flavour (HFL). esotic and rare
phenornena (EXO) and muons (SILO). There are also an additional two slots. VTSO1
ahich selects a control saniple of events with a reconstructed vertes withiri I;,,, / < 7.5 cm.
and SAP01 which selects events tagged by the leading proton spect ronicter.
The physics filters niake use of al1 the reconstructecl data wbicli takes pliice iit t tie
TLT prior to the events being passed through the physics triggers. In particulnr. S C DIS
events rely on the e= identification. Four separate algorithnis are iised at the TLT. The
LOCAL algorithm searches for clusters of energ- deposits in the calorinietcr and c m on the
ratio of ESIC/HAC energy [XI]. E L E C T ~ [ ~ ~ ] siirns the energy in a 1 1.2" coiie iiroiiricl EhIC
cells. The other two algorithms run online are more sophisticatcd. S I N I S T R A [ ~ 1. 521 is a
ncut r d net büsed finder and E M I L L E [ ~ ~ ] iiscs probability rlist ri biit ions of ( k t ector rclsponsrl
for e' and is a variant of the e=-finder usect in this thesis (sce section 5.4).
.Jet-finding algorithms (see section -5 .5 ) are also iised onlinr. Two jet-findcrs. E U C E L L ! ~ ~ \
and KT. are employecl online. The latter is the one iiscci in this analysis.
The filter dots are the responsibility of the physics groiips at ZEUS. Tlir pritircl TLT
filter code can be simulated offlirie tising titzgana. This catie is a ~ î i l a b l c to al1 physics
groups and is used to verif? online trigger decisions aiid devdop or ripgrad~ filtrr slots.
Cnlike the FLT. a trigger d o t is a software fabrication aiid th^ riiimber of dots is not
restricted and siniply depends on the processing poiver required to nicet the tleniantls of
the esperiment.
Performance of the TLT
The TLT has been a crucial part of the ZEUS DAQ chain since the inaiigural riin in
1992. In 199.5 the TLT hardware was redesigned ancl the old Ferniilab Brandi Bus
system[55] was replaced a i th more modern and commercial technology. The writing of
events from the PME-FDDI-1 board to the workstations operates at a siistained rate of
5.S 'rlByte/sec [56]. The hardware itself is able to mite at a rate of 7 'rIByte/sec from
Processing Time irns)
Prncessing Time (ms)
Processing Time (ms)
Figure -4. I-L: The CPU processing Lime rerpired by the TLT. ( a ) Fm- R4400/150 .CiHz
processors of which there are 7. ( h l /or R4400/250 M H z pîncessor.~ of ruhrch tht:rr~ trrr 7
and (c) for fi 10000 processors OJ w h ~ h there ure 5 used online.
its interna1 DR-ILI to the FDDI loop. but is limitecl by tlir data transfer spwd ovcr ttir
VSIEbos. Event transfers from the workstations to the archiver merr nicasiirrd[Sûj t» Iir
11.5 SLByte/sec. With the EVB writing out at rates of up to 30 Hz per YUE riierriory ar
an average event size of 100 kBytes. one PME-FDDI-1 board keeps up reaclirig oiit two
mernories. yielding a 60 Hz event rate per TLT branch.
Over the years more demand has been placed on the TLT to make niore estensive
event reconstruction in order to provide more refined and sophisticated trigger decisions.
CPC power has been added to meet this demand while maintaining the design reqtiire-
ment to handle 100 Hz input event rate. In the current configuration the input limit of
the TLT has been estimated to be 103 Hz[57]. Typical CPC' times are shown in figure
4.14. The distribution of total processing times in 1997 indicate a mean processing timc
of 237 ms. which is dominated tq- the certes and track reconstruction processing time.
The system is extrernely resilient. allowing processes to restart autoniatically tluring
data taking. -1 process may crash for several reasons. For esarnple. a harclwarr fail-
lire whicli could be repaired niid-run. a power glitch. meniory resources rnx~iniized by
system processes causing a reboot or network interruption caiisirig rriiiltiplc tinieoiits.
The physics analyzer process rn- also crash due to a rare bug in the filtctr (*O&. Ii i
tliis situation. the event which was being processed tluring tlic crash is tliirnpcd to ilisk.
allowing the TLT expert to retrace the cause and remetly aiiy prograrnrning anonialy.
The communication processes are al1 based on standard network p r o t o d s whirli tln-
sures good portability and maintainahility. An- deniarid for more baridwidth or CPC
power cari easily be satisfied by adciirig niore TLT branchtas and/or iriorp ivorkstiitions
to thc system. Tlie separation of the filter layer and ot1it.r TLT proccssrs iilloi~s for ilas!.
modifications of the physics algorit hms.
Chapter 5
HERA Kinematics and Event
Reconstruction
5.1 Physics at HERA
5.1.1 Deep Inelastic Scattering (DIS)
Lepton-Hadron scattering h a s bren a ricli soiirce of information iri the p s t for iindor-
standing the structure of nuclei and nucleons. HERA continu~s ttiis tradition I>y prohirig
the structure of the proton a t srnaller distance scales than e w r before. The process of a
positron-proton interaction is showri in figure .?.l to lowvest orcler in perturbation theor?.
4
Giren the k e c t o r s of the initial ancl final stntes. C = (E,. k). p = ( E,. li) a id
kt = ( E t . k'). J = (Es. J ) respectively. one can derive four Lorentz invariant qiiantities
which characterize DIS. The interaction is characterized by the constant centre of mass
energ . 6. which is a function of the energies of the colliding beams:
CHAPTER 5 . HERA KINE~IATICS AND EVENT RECONSTRUCTION
Figure Ll : .4 schemalic uiew of the Deep Inelastir Scatt~nnq Procrss r ~ t HER.4.
Sotr that at HERA bearn energies ttw beani particlr ttiasscs c;rii h t > ric~gltv:trd in t , l i ~
derivation of .S. The other invariants arc:
Q2 is the negative of the Cmomentum transfer squared betmeen the initial and final state
leptons and sets the scale of the interaction. At high Q' the positron-proton interaction
is essentially a positron-quark interaction. The invariant s is the fraction of the pro-
ton's momentum carried by the struck quark. Finally. g. sometimes referrecl to as the
inelasticity. can be interpreted as the fractional energ' transfered From the positron to
the proton vertex. in the rest frame of the proton. The scattering angle in the centre of
CHAPTER 5. HERA KINEMATICS AND EVENT RECONSTRUCTION
mass frame: O * ? is related to y via:
The kinematics of this process is over constrained by r. y. and Q'. since they are related
by the equation:
Figure 5.2: .4 xhematic viem of the s-channel resonancri production nt HER.4.
This thesis describes the search for a scalar porticle s-channel resonancc pri>~liiced as
showri in figure 5 .2 . The dynaniics of production of such a particlc arid its siibsrqiirnt
decay are discussed in section 3.4.1. The mass of the resonarice can be espresseci in the
following way:
.112 = (k + XP)' zz rs . ( 3 . 7 )
One other important esperirnental feature which is used in this thesis is the variable
6 = E - Pz. where E and P, are the total energy and the total longitudinal monientiirn
of an event. Since the ZEUS coordinate system defines the positive 2 (fonvard) direction
CHXPTER 5. HERA KINEMATICS AND EVENT RECONSTRUCTION 62
to point in the direction of the outgoing proton. energv and longitudinal momentuni for
forward moving particles are essent ially equal. Hence t heir contrihii tion to t j canceis tu
zero. Particles moving in the rear direction have energy and longitudinal rnoriientiint of
opposite sign and thus their contribution to 6 is twice tlieir energv. 6 is a conserveci qiiaii-
tity and hence if al1 possible particle losses in the forward and/or backward beaniholes
are ignored. b can be caiculated from the initial state and miist be eqiial to tliat from t h r
Aria1 state. By conservation of rnomentum and energy the difference betivi.cn thc total
energy and the total 2-component of the monienturii is qua1 to twicr the initial elertron
bcarn energu:
Sote that particles which escape detection in thr forwarcl bram holr \votild giwri urily ;i
very small contribution to E - Pz.
Seutral Current (SC) DIS occurs through the exctiaiige of a photon or Z0 iind is
characterized by a positron in the final state. Chargetl Ciirrent (DIS) occiirs throiigh
the eschange of a IL'' boson and is characterized hy rnissing transvcrsr niorrii.ntiim (Pr
due to the escape of the neutrino froni the detector. The S C DIS cross section drops
like 1/Q" and the CC DIS cross section (s - ( 4 1 L \ , i . ) 1 ) falls off e w i faster due to the
rnass of the II' boson. .C i i v . in the cross section formula. This analysis is concerned witli
events which would populate the kiriematic rcgion of high Q2.
At Q' - 0. the exchanged boson is a real or quasi-real photon. in a process callecl
photoproduction. In such events. t he scattered positron disappears into the rear beam
pipe and cannot be measured in the main detector. Photoproduction events forni the
rnajority of HERA data.
DIS and photoproduction constitiite the backgrounds for the physics bring searchecl
for in this thesis.
5.1.2 Reconstruction of Kinematical Variables
Table 5.1 defines the notation usetl in this thesis. Polar angles arc rneasurcd witti rrspwt
to the z-axis.
El Energy of the final state positron
8, Polar angle of the final state positron
S Energv of the struck quark
Polar angle of the struck quark
1 E, Constant Energy of ttie initial positron ( 2 7 . 3 G d . ) I 1 Ep Constant Energv of the initial proton (8.10 GPI.) /
Table 5.1: ;Vatution.
The scattered positron in ttie final state provides a tiariclle For recogtiizirig S C DIS for
triggering and reconstructiori. The clectrori niethod[5S] for reconstructing t h kincrnatical
variables defined in section 5.1.1 makcs use of the scattered positron's rnergy ( E ' ) and
angle ( O ) :
Q T ~ = 2E,E1(1 + cos O , ) .
E e E'(1 i cos O,) z,i = -
E, 2E, - E'(1 - cosl),)'
The kinematic variables can also be defined in terms of the angles 0. and 7 . This
method, which is the method of choice in this analysis. is aptly called the double angle
CHAPTER 5 . HERA KINEMATICS AND EVENT RECONSTRUCTIOF~
met hod[ZS]:
sin 3( 1 + cos 19,) QL, = 4 ~ : sin + sin 0, - sin(& + 7) '
E, sin y + sin 19, + sin(& + 7 ) XD.-I = - Ep sin -/ + sin O, - sin(& + î ) '
sinO.(l - cos r ) !/D.-I = sin 7 + sin 19, - sin(& + - 4 ) '
This met hod lias the great advantage t hat it is insensit i w to iincwtaint ies i r i t lie c:aloritii+
ter encrgy scaIe to
cari bc shown that
first order. The problerii lies iri
7 can be calculateci via
ttie cleterniiriation of t h ariglr. It
P;,h - ( E - C'OS 7 =
P&, + ( E - Pz) ; '
The .Jacquet-Blonclel metliod[59] i1st.s the haclronic criergy Horv of an twv i t to rwon-
striict ttie kincmatics:
nhere Sh is a sum over the hadronic energv in the event. The difficiilty of this method
is in the determination of S and 3 . One does not esperimentally directly mensiire thesr
quantities. In this thesis. jets are used to reconstruct the hadronic final statr ancl hrncc
the summation indes on Sh is the number of jets. Despite ttiis tlrawback. the Jacqiiet-
Blondel method provides the on- tvay to reconstruct the kinematical variables for CC
DIS and photoproduction where the scattered lepton goes undetected.
CHAPTER 5. HERA KINEMATICS AND EVENT RECONSTRUCTION
5.1.3 Reconstruction of Global Quantities
The following global event quantities are essential in the event selection. The longitiiclinal
cnergv variable given in equation 3.10 is clefined as lollows:
EL = ,/i;, + Pj;, + Pf,. (+5 -22 )
The f i . referrecl to in the previous section. is magnitude of thc vector siirri of thcx
transverse momcntum and is calculatecl by
The sunis i are over d l calorimcter objects. The srriallrst of siich caloririwter o t > i t ~ ~ s is
a calorirrietcr cell. Hoivever. cclls cari lw cl~istcred togctlicr to for111 islaiids or j r ts . w l i i c 4 i
arc tlescribed in folloiving sect ioris 5.4 and 5 - 5 . and t tic. above variables cari (ittfiricd in
terms of t hose exteiicled objects.
5.2 Track and Vertex Reconstruction
V C T R A K [ ~ ~ ] is a fortran package which finds tracks. the primary vertes and sccondar-
vertices for ZEUS events. Each reconstructed trttck makes use of data in the CTD
although information from other trackiiig devices can also be esploited. For the purposes
of this thesis the regular CTD-only tracking and vertexing version nas used. VCTRAK is
a component of the offline reconstruction program and an optimized version of the same
code (limited to CTD only information for purposes of speed) is tised oriline to do the
vertex and track finding riecessaru for the onliiie trigger decisioiis.
Tracks are first reconstructed in the ( x . g ) plane and tlien cstended to t l i ~ : diniension
using the 2-bu-timing and z-stereo information from the CTD. Track candidates b ~ g i n as
a seed. consisting of three hits on an axial outer Iauer of the CTD. and arc swuni inward
towarrl the nominal centre of t = y = 0. -4 Fourth rirtual hit is ackled kit th(! bmiti
line to aid the track as it is estrapolated inwartls gathcring additional hits resultitig iri
increasing precision of the track pmnir t r r s .
The primary vertex is found by first removing ariy tracks not corripatible witti origi-
nating on the heam line. A n initial simple vertcx fit is ttien performed on the rcniainirig
tracks which assigris a tveight to the (x. y. 2) origin of cach track. A secorid iind rriurr
cornprehensive vertes fit is finally prformed. in whicli the directiori and ciirvatiirp of
each track are adjiistccl to the final vertex position.
5.3 Corrections
In the determination of event parameters t ht. energy rriraurement and its reproduci bili ty
in the MC are essential. Energy corrections are necessaru since particles originating frorti
the e p collision traverse different niaterials before reaching the niain calorirtiet~r. Thi.
measured energy in the calorimeter would thus bc rediiced by the lost aricl iinrccov~rahlc
energy wit hin such material. Ot her reasons for energy correction include noise. back-
splash and shifts in the calorimeter position between years of running.
Calorimeter Noise
The ZECS calorimeter uses uranium as an absorber as dcscribed in section 4.2. The
natural radioactivity of the uranium creates noise in the calorimeter and is rneasu r~ i
CHAPTER 5. HERA KINEMATICS AND EVENT RECONSTRUCTION 61
dong with the energies coming from the ep interaction. This noise is suppressed bj- aii
energy cut of 60(110) MeV for al1 EMC (HAC) cells [60. 61. 621.
CAL shift
.\ rnensured discrepancy between the BCAL and RCAL ce11 positiuris \vas obscrvrd hi.-
tween tlie data and SIC and is corrected duriiig reconstructiuii. Ttie BCAL is shifted iri
the :-direction by +6.5 min[63] and RCAL is shifted by +1.1 crn[G4] in thr xlirection
from 1996.
Energy clustering and Island corrections
Ueasurement of global energv variables based or1 cells are correctcd by factors wtiicti
are pararneterizeci as a function of some global quantities. usually thr g loM enrrgic1s
thcmselves. Such a global correction is perfornied for the hadronic energy deposits in thi l
CAL and corresponds to +5% in BCAL and +Xi% in RCAL (C.-\LCORR[65]).
;\ri improvenient iii energy resoliitiori can brb achicved i l a (:orrrctioii i v r w cloiit. for
eacli particle 1661. Since each particle coming from the ep interaction can (lrposit rnergy in
more t han one cell. clustering of energy deposits start irig frorri c:c?lls is prrforrrid. Thosr
clusters of calorimeter cells. called islands. gattiered around a single local niasiniiirti
approsimate a particle shower in the calorimeter.
These clusters are the islands which were briefly mentioned in section 5-4. The cliis-
tering is performed separately in cach of the F/B/RC.\L aricl siibscquent nirrgirig is
performed between islands which are deemed to be coiinected or traverse two CAL sec-
tions [67].
One source of deviation from the true energy is loss in inactive material between
the interaction point and the surface of calorimeter. -4s hadrons and electrornagnetic
particles are expected to behave differentl- the islands are split into two categories and
separate island corrections are cleveloped based on dead material rriaps for tlio diffcrent
years of ZELS operation 168. 691. Corrections are also performed for energy lossrs in tbt.
super-cracks betweeri the F/BCAL aiid t lie R/ BCAL srct ions.
Another source of correction arises Froni back-splash of particles rebouriding off the
face of the F/BC.-\L. This so-called albedo effect is illustratecl in an event display of a
CC MC event in figure 5.3. Albedo causes an enhaiicement in ( E - Pz),, and thus in ttio
hadronic angle. -,. in equation 5.17. This can have a significant effect on the absolute
value and resolution of the kinematic variables. an effect on (E - Pz) of O( l ) GeV [69].
CHAPTER 5. HERA KINEMATICS AND EVENT RECONSTRUCTION
Regularizat ion
The back-splash and energy corrections were not derived with the assuniptions of an-
kinematic constraints. In particular. the following constraints w r e not iniposcd:
-4s a resiilt resolution effects ciln cause kinematic variables to bc calculatecl. ilc.cwrtling t o
the .Jacquet-Blondel method, with the previously tliscusseci correctioris applied. esceediiig
t heir liniits. A procediire was developcd aloiig wi t ki t lie correct ions (corandcut [69]) t o
restrict the Jacquet-Blondel variables to their kinematic limits. Thp dh and P-I-.h \ariablt's
used in the subsequent event sclection cuts are not regiilarized. Figiirc 5.4 illiistratrs the
resiilt of this regularizntion on th^ ! / J B variable for it samplc of CC .\IC t v n t s .
5.4 Electron/Positron Finding
.A crucial aspect of t his anaiysis is the clectron/positrrm finciing. For t«pologirs w h i d i
include a final state e= t he need is obvious but in the case of a neutrino in thc final
state the observation of a e' candidate is iiseful in the rcdiiction of backgroiinrls. H m w
correct and efficient identification of e I and precisc r~constrtiction of its position x i d
energv are of vital importance to this search. The r~construction algorithm i.ised in t liis
analysis is called em and is describeci in [33]. The em e' finder is based on detailed
parameterizations of the detector response for elect rons/ posi t rons. Several variables arc
iised to create stib-probability distributions for the calorinieter and CTD. Finally tliese
sub-probabilities are cornbined into a grand probnbility distribution which cari be uscd
as a selection criterion.
The outline of the algorithm is as folloivs:
1. Cluster the calorimeter cclls into islarids.
2 . Loop m e r islands and accept an islancl slioiild it pass the following critt.ria:
The energy of the island is at least 4 Ge\*.
The energy fraction in the HXC section of the caiorini~ter rniist t ~ e lcss tlian
0.3 in FCAL and RCAL and less than O.: in BCAL.
0 The calorimeter probability must be greater than 10-?
3. If the polar angle of the island candidate satisfies 0.3 < H < 2-85 tlieii check For a
matching track. Tracks must satisfy the following criteria to br considcred:
CHXPTER 5. HERA KINEMATICS AND EVENT RECONSTRL'CT~ON
0 The distance of closest approach of the track to the bcarnline niiist he less
than 2 cm.
0 The distance of closest approach of the track to the islanti niiist bc less tlian
50 cm.
le-- - ~ ' r l f i n d 1 < K/-!
Io truck - oislandl < I i / d
The tracking probability m u t be greatcr tliari 10-:'.
4. The grand probability must t ~ e greatcr tliari 10-".
Figure -5.5 shows the performance of the EM findiiig cfficiericy cornparcd to tliat, of
a neural network Finder SINISTRA! The findi~ig algoritlinis arc riiri uri NC DIS iiticl
photoproduction MC simulations. It should bc noteci that t h r EM finder siiffers Frorri
a slight inefficiency a t the regions between thtl CAL sections. Ttierc is &O ;i drop in
eficiency outside the CTD acceptanw as therc is no trnckirig inforniation to siipplrnit~rit
the grand probability determination. In the FCAL and BCAL rcgioris. wit h t tic escept ion
of the region inbetrveen these ttvo CAL sectioris. the EM tirider lias a an rtficieiicy o f o w r
97%.
5.5 Jet Finding
.Jets are an abstract construction used to relate measurements made a t the dctector level
to a primary particle of a hard scattering process. -1 large portion of the final statps
arising h m a potential squark decay contains multiple quarks. Jet couriting is central to
this analysis and will be discussed further in chapter 7 wtiich details the erent selection.
'SIRA 5 is a particular implementation of the SINISm finder.
All events
Figure 5.3: The positron finding efitxicrer~cg us r i furtctcon o f erceryy. polar. u r ~ g l r t r i i d
10~,~(Q')[70] O/ the true positron /or Eiçl (crosses,) and SIR.4 5 (diumonds). The rn-
ner vertical lines indicate the CAL limits and the outer vertical lines indicate the CTD
limits.
The number of jets may not directly correspond to the nurnber of partons resiilting frorii
a squark decay A lower number of jets than initial partons can be reconstructed when
the energv deposits from two or more partons are not well separatecl spatiallu arid ruorp
jets than partons may be reconstructed diie to higher order QCD eff~cts. The latter is
the source of background for the niultijet States.
The kinernatics of the jet are defined by the following equations:
wtiert. E;~. r f r t and de' are t hc transverse imrrgy. pseiidorapidit!-' and üziriiiit lia1 angk
of the jet respcctively. The sunis can be perfornied ori ilriy caloriirierrr ohjrcts. froni cc4s
to islands which bdong to a particular jet. Ir1 ttiis ;inalysis. ttir jet firiding is prrforriicvl
on the correcteci islands.
The jet reconstruction algorithm used in this thesis is the longitiiclirially inuriant Kr
algorithm which is described iri the literature [il. 721. The algorit hm works hy ralciilat in8
a distance measure.
d2 1 J = (Ar$ + ~ ~ ~ ~ ) r n i n ( ~ - ~ ~ . ET]) . (.5.:30)
and
for each pair of objects. The minimum of (d$ $) is fourid. If the niinimcirn is cl:. then
the object i is removed from the list and called a jet. Ot herwise the objects i and j are
merged by the ET weighted recornbination sclieme given in equations Z.27-5.29.
' q = - ln (tan 4 ) : where 0 is the polar angle.
5.6 Squark Mass Reconstruction
The squark mass reconstruction met liods use reconstriicted jets. whicli take the cor-
rected islands ciescribed in section 5.3 as input to the jet algorithm. The longitiiclinal
conservation of energy in equation 5.10. b = 55 GeV. is also assumed in thcir derivation.
5.6.1 Channels with a ei in the final state
The tnass of the squark is reconstriicted as folloivs:
mhere ( E + Pz) is calculated iising the cz candidate arid the rcconstriicted jcts arid E,. is
the initial e-beam energu.
5.6.2 Channels with a u in the final state
The PT is attributed to the u in the firial state sucti t h :
meus -Pr.,
Ev = dl$., +$:.p. (3.323)
The right hand side of eqiiations 5.34. 5.35 and 5.37 are measurcd. Since the right haiid
side of equation 5.37 is a
a relation for p,,, cari be
constant. A. for each event. thcn froni equations 5.37 and 5.35.
derived as follon-s:
- Jn - -4 Pt," - Pl." + PZ,"
Herice the mass of the squark c m clerived:
Siibstituting eqiiation 5.49 into 5.45. the invariant nii~ss o f thil v and jets systrrri riin br
exprcssed in terms of measurable qiiantities.
where (E + Pz) is calculated using the reconstructed jets and y is regiilarized. as tlescrih~tl
in section 5.3. to be less than 1.
Chapter 6
Monte Carlo Simulation
Slonte Carlo (SIC) sirniilations play an tscnt ial role in plaririing and anelyzing tiigti m-
ergv scattering experiments. It allows for the desigriing of selection ctits arid calciilarion
of the acceptances. A Slontc Carlo generator simiilates pliysics bu cr~atirig ~wiits aword-
ing to probabilities of the desired process cross s~ctiori mt i proviclcs evrnt distxil~litions
basctl on an iincierlying theor-. Sincr thil espt.rirrieritii1 s c ~ ~ p is not prrfrrt iiritl i r n p w s
cuts in phase space. which are in general qiiite conip1ic;itccl ilricl ciirinot btl trratrd i i i i -
alytically. MC techniques allow for the stiidy of physics processes on a statisticd ba i s .
Generating events is but the first step of the coniplete simulation as other prograriis arp
siibseqiiently used to siniulate the interaction of t tie firial state particles mit ti t tic dctector
and the behaviour of the triggcr.
6.1 Background Simulation
The dominant background to e'q. e'qqq. and eiqqqqq final states is SC DIS. For the
e'q final state. the background is due to hard 2 i 2 scatters between the high-+ quarks
and the positron. Backgrounds to the multijet final states occur in NC DIS events mith
Luminosity ( 1 1
CC DIS Q' > 10 GcV' 24742 -596.0
Table 6.1: Surnmapj O/ yenemted buckyro~irid Monte Curlo .surnyle.s.
Events
Gencrated
Generator
Q' > 5000 G ~ V ?
Q2 > 10000 GcV2
Q2 > 20000 ~ e \ "
HERKIG L
higher order QCD effects. The background to rop«logies containing an P - is qiiitc srtiall.
since it occurs only if the positron charge is ivrongly reconstriictecl. ;\clditional sniall
D.JASG0
Process
148-42
-4981
49 1.3
PHP Direct
backgrounds can corne from ptiotoproduction ( y p or ptip) processes if a fake positroii is
Generator Cut
131234
24642
11641
1 1894
.j990.8
10396.1
l.jllü9.2 j 4
I PHP Resolved , P:("* > S Ge\-. ET > 50 GPY i 14886 !
P:"'" > Y G d . . ET > 50 G r \ - 1 9839 38.j
identified in the hadronic final statc.
SC DIS ' 126.8 1
1:36-4 i l
2 19.0
912.8 l
115.9 1 !
The prirnary background to v multijet topologies is CC DIS with again sniall ad-
Q' > 400 G ~ Y '
Q' > 1250 ce\*' Q' > 2500 Ge\-'
Q' > 5000 G ~ Y '
ditional background from photoproduction events for wliich the riieasured transwrse
momentum is large due to mismeasured jet energ- fiiial state muons or neutririos.
The background samples considered are summarized in the table 6.1.
HERICLES -l.fi.2[73]. which simulated elect roweak radiative effects. interfaced wit h
DJANGO 6.24[74] to hadronization programs aas used to simulate the backgrounds
Figure 6.1: Examples of leading order dia gram.^ /or (u) direct czrrd Ib) resolued plrotopro-
duction processes. The proton remnunt is lubelled R.
coming frorii S C and CC DIS. The hüdronic fiiial statr is siniiilatctl iisirig ttir Color-
Dipole Uodel as implemented in ARIADSE[72] for ttic QCD cascade. Thr CTEQ-I[X!
parton density parameterizations were iiseci. Ttie partori densitits are an ttssrritial pi r t
of the cross section calculation ,as in the cross section equation 3.11.
Photoproduction events aere simulatecl using the HERKIG gencrator $Ti. ;\ dist in(--
tion is made betwvem twvo types of photoproduction. direct and recolved. botli o f which
were simulated and illustrated in figure 6.1. In the direct case of photoproduction the al1
the energy of the photon participates in the hard scatter whereas in the resolved case only
a fraction of the photon's momentum. associated a partonic constituent of the photon.
participates in the hard subprocess.
6.2 SUSY Simulation
Over -LOO signal MC sets were generated using a modified version of SPYTHIA[3] and
SCSYGEN 3.0[78]. The decay q' + eTq was sirnulated ivitli PYTHIA 6.2[79]. These sets
are summarized in tables 6.2 and 6.3.
In an attempt to cover tlie SCSY phase space. several points were selectcd to grnerat(.
as they encornpassed the relevant gaiigino n i a s sp;we. as illiistratccl by figiirc 6.2. .Ar
each selected phase space point. each topologv \vas sirriulated a t ~~l l i i l rk niasses ranging
from 100 GeV to 280 GeV. These simulations allosecl the detcrrniriation of tlie signal sr-
lection efficiencies. The efFiciencics ;ire sensitive to the masses of t hc part icles potcnt ially
decaying in the ZEUS detector. Hence. by simulatirig eiicii topology listed in t 'abl~ 3 . 3
for several squark masses. one (*an determine thr eficiencics as a fiinction of the sqiiiirk
mas. l l o r ~ o ~ e r . by sclccting points iii the iriiiss pl;lncbs ( < y . < ; ) arid ( ï:. iy) o i i r b m i i
indirectly scan the SCSY parameter space of ( J I 2 . p. tan 3). Rwall tliat t l ip niasses of
the gauginos are fixed by a particular choice of .l12. IL and tan .j. Strategically rrhoosing
points to lie on the extrernities of the possible n i a s States pmri i ts the intcrpolatiori o f
efficiencies at non-sirnulated points in the SCSY p t i a s ~ spacc. Hcrice. a scari in the SCSY
phase space can be perforrned.
SPYTHIA
SPYTHIA is a subset of the PYTHIA 6.2 generator. SP\THIA simulates particle pro-
duction in the SISSM and does not inclilde ie, processes. 'rloreover there is no prowss
for sparticle production at ep colliders. Fortunatelu. PYTHIA provides several ways for
including new processes and/or riew particles. PYTHIA contains the code to siniuiate
the production of a scalar leptoquark through the fusion of an e- and a quark in ep
collisions. This is the same process required to produce a squark via & at H E M . This
Generator JIP Range Topology
1 SPYTHIA 2.2 1 190 1 -200 1
.-
1 SCSYGEN 3.0 1 100 1 -60 1 100-280
/ SCSYGES 3.0 1 130 1 -200 1
l 1 SCSYGES 3.0 / 310 1 -140 1
Table 6.2: Sumnranj O/ generuted signal Monte Curlo .sumples iuith multijet final states.
Table 6.3: Summay of generated signal Monte Carlo samples with single jet final states.
ecl
1 (20 Ge\' intervals)
PE'THIA 6.104 100-280
Figiiro 6.2: Ench srnnll dot reprrisents n pornt ln (1 g n d t h c h covers the (J-. p ) rpyzon
plane and (b) in the .\ffT us .Ilp plane.
process is used and as PYTHIA facilitates the inclusion of riew d e c q niod~s. ttic clrcys
to SCSY particles are added. -4s SPE'THIA conteins only .\ISSII decap the LSP wis
forced to be stable by setting the particle's width to zero. This au bypassed in the
routine which initializes the SCSY sector of the generator.
Al1 relevant information. such as widths. lifetinies arid branctiing ratios must be pro-
vided in the form of supplementary particle data used to define new decays. This can
be done using any number of other programs which calculate the needed information.
SGSYGEX- which is briefly described in the following section. was used for such sepa-
rate information. No special matris eleinent treatment code is providecl to perform tlie
& decay The decays are performed via phase space. The cross sections clo not take irito
SUSYGEN
The SCSYGEN pachgc. initial1 y written to sti
account the interference between different processes[3].
y SCSY evrrits in e'e- (:dl'
estended bu E. Perez[80] to cover HER.4.s processes. Likc S PITHI.4 nlrnos t any prowss
can be generated excepl t h it perfornis al1 two or three body SCSY clcc-s accortiirig
ta full matris elements. The matris elernents for eacli harcl siibprocess arc giwn in [Z].
SCSYGEY provides useful routines to calculate the spectrurn of SCSY particle niasses.
dcc- midt hs and cross sections. .Usa t-(il) chnnnrl slrpton (sqiiark j escliarige i i i t hth
eq + q i ';: process is incliidtd.
Both SP'I'THIA and SCSYGES iniplenimt initial ii~i<l firial statv partons sliowrs
which have the effect of broadening t hc mass distribiit.iori -11, ( JI, = J-C;;) of the sqiiark
as seen in figure 6.3.
Severtheless. given the widths and branching ratios. SPYTHIA calctilat~s t h . cross
sections in agreement [3] with SCSYGES. ;is sliowri in tigurrl 6.4.
6.3 Detector Simulation
The generated events were input into the ZELS detector simulation callcd hIOZ.-\RT[29]
which is based on the GEANT 3.13[Sl] CERS package. It simulates the passage of parti-
cles though al1 the components of the ZEUS detector. Finally. the trigger is sirnulatecl by
the ZG.ANA[82] package. The full offline reconstruction of an event is performed by the
software package ZEPHkX[29]7 nhich takes al1 calibration constants into account and
treats data and Monte Car10 simulation in the same way.
Figure 6.3: Eflect of initial and final state purton sho.wers on the mass recorist niction O/
a 200 Ge V quark undergomg the P, decay (i + eq . ISPS c~nd FSPS refer tu rnitznl und
final d a t e parton showers respectiuely.
- Figure 6.4: o(e+d + q y l ) as a funct~on o j the SUSI' parameter p. Block points r i - /PT- t»
SUSYGEN and open stars refer to SP YTHI.4. Both p l o t s keep SIG = 200 Gr C' tarr.j=L. U .
Above: .& = Z O Ge V. Below: JI2 = 100 Ge Ca*.
Event Select ion
The analysis focused on two classes of events. One class contairis o. rrx-onstriictrd electrori
or positron and hence will be callecl SC-like. In th r other clas of rvcnts. thr final st.attl
lepton is a neutrino: thiis the signature is characterizcd by niissing txansvrrsr rriomrntiirn
(Pr ) and this class will be called CC-likt.. In eithcr rasch. one is s~arching for a higti niass
resonance and thresholds for cuts on Err and f i (:an hc set higti. sincr high thr~sholtls
would not affect a potential signal nliile significant ly rediicing backgrotiiid cont;iniinat ion.
7.1 Search Strategy
After selecting high Q' S C and CC DIS topologifs the let reqiiirernents are ttim of'
utmost importance. The- provide the greatest discrimiriant between the signal and major
background sources. The cascade squürk decays contairi t h r e ~ to fiw quarks whirh at the
detector level translate into a sharp peak at tno or niore good jets (figure 7.1). SIoreorcr.
the jets have high ET. whose mean increases with increasing squark m a s . A good jet
is defined to have q < 2.5 and ET > 10 Ge\;. The NC and CC events peak at one
reconstructed jet and fa11 sharply as shown in figure 7.2. Hence. the key cut in estracting
Figure 7.1: Above: the number of good jets reconstn~cted in MC sirriulatzons /or ( ( I I the
eqqq final state and ( b ) the eqqqqq final stute. B e l o i ~ ~ : the Err distnbutzon of the hqhest
Er jet for ( c ) the eqqq Jnal state and ( d l the eqqclqq final state. The dashed histogrnrn
represents squarks iuzth a mass .Ili = 160 GeV a n d the solid histotjrarrr represerrts squurks
with a mass = 260 GeV.
P -1 ' - 7 * 4 l0 O - 7 4
Num. good jets Num. g o d jets
Figure 7.2: T h e nunlber of good jets reconstructed rn .CIC .szmdat~oii.s for .VC «nd CC
DIS are s h o w in ( a ) and ( c ) respectivel?y. The ET distributions of the hiyhhest ET jet /or
NC and CC DIS are s h o m in ( b ) and (d) respectivel!y.
O - 7 3 Num. good jets
Figure 7.3: ( a ) The rrurnber of good jets /or urc rq f i r d stntt. ( b } The ET oj the highcst EPr
jet foi. un eq @al state. The dashed Iii.stogram i-ripresertts .sq,irarks of rriavs -\I,j = 160 G d '
und the solid histogmm represerits squnrks of rrru.s.s = 260 GtK.
the cascade squark tiecay topologies is tu require at leest trvo well reconstr~ictrcl j ~ t s o f
significant ET.
One ivould espect a peak at one good jet for direct 4 squark decays (see figure 7 .3 ) .
since there are only two partons in the final state: a positron and a quark. This is the
same final state expected from NC DIS. Howeïer. the ET of both partons d l have higher
ET t han t heir cascade counterparts.
7.2 Cut Optimization
In the following event selectiou. described in the next sections. some cuts are said to be
optimized. The optiniized variables deterniine thr final cffiriericy. c. of a signal. The! arc
set to rnavirnize the acceptance of the signal while miiiiniizing DIS backgroiiritl s« that
the best cross section limit. o~,,,,,. is obtained. Thc \-due of oit,,,( depends on the nuriiber
of data events. However. in deriving the cuts orle rriust riot be biasetl bu lorcknowltlclg~
of the data distributions. as one may formulate a cut to enhaiict. or siippress a possible
signal. The chosen variables must be optimized using only llontc Carlo sinidation.
where Job, is the nurnber of observeci tavents arid p is t h t~ nuriit~er of cspecteti tlvcrits frorii
backgroiincl Monte Carlo simiilat ion. Ttie nieiin liiii ir is a Poisson-ivrigtitd si1 t i i i3t.r.r ;il1
possible .Vob, and is independent of the data. Thr nr~,,l,l(.Vob.) is the cross sectiori litriit
which is discussed in chapter 8 and defincd in equation 8.5. Ttiis proccclure is repeatecl.
for the cut variable to be optimized. for rach scpark rnass. resulting in a selwtion ctit
which is dependent on the reconstructed invariant mas in the detector.
The SC-like events are selected primarily by the identification of a final state electron or
positron and secondly by high global Er. At the nest level of selection the subsequent
data set is segmented into rnultijet and single jet events. Finallx the charge of the ex
candidate is used to partition the remaining sarnple.
7.3.1 Preselection
.ifter events survive the three levelled trigger system. a process of reconstructiori tiikrl~
place. At this stage a new level of filters are applied to the data and the rcsults arc
classifieci in Data Sumrnary Tapes (DST). These DSTs form additional bits wliich can
be iised as a first selection or presclwtion recpireriimr in offliric arialyses. Tlie Following
selection cuts form the first p a s over the data to select iveil reconstrricted events of high
ET -
DST (33 or 36) and 35 - - -
DST bit 35 selects events with ET > 20 G d - ~ ; ~ l ~ u l a t r ~ d from islancls.
DST bit 36 selccts cvents rvith ET > 30 G d - (:alciilatecl fror~i tlw CCAL crlls.
DST bit 35 selects pverits with ERC..IL < :> G d . c*al~~il ;~t 'c l from th^ RCAL cdIs.
Vertex requirement
A tracking vertex is recpired. which rnust satisfy lZL,t,l < 30 cm. Ttiis corifiries t l i ~
vertes to t lie central region of the cletector.
( E ~ ) u n c o r r e c t e d > 40 GeV and ( E - P:)uncorreded > 30
.A preselection is niade for events with high ET and tiigh E - Pz in order to prefer-
entially select high ET NC DIS rvhile removing a large fraction of photoproduction
( p hp) . These variables are calculated from the raw calorimeter energies wi t hoiit
any correct ions applied.
CHAPTER 7. EVENT SELECT~ON
7.3.2 Further NC Selection
Having selected a high ET event sample. the foliowing cuts were applied to select higli
SC events:
( E - Pz) > 44 GeV
-4 liarsh cut for removing photoproductiori.
X yood elect ron/positron foiiticl
- E, > 15 GeV with an em probability > 0.001
- E,,.(R = 0.25) < 1 GcV
The candidate miist be isolateci. Thc wiergy insidr a cbonr of r:itliiis R =
JLq2 + LO' = 0.25 in ( q . 0)-spacc not attributrd to ttic rlcctromagnctir rliis-
ter should be less tlian 1 G K . This is a less strict isolation critcria thiiïi t . h t
used in a standard S C DIS P = search (set. appcndis A.?) b~cai i s r in a sqiiark
cascade decay the P' COCI~CS fror~i a rriultip~rtoti srcondary drcay of ii gaitgino.
- 8, > 0.3 rad and a rnatchirig track with PFf1'" 2 GGe wiiich has a ciistanw
of closest approacti (DCA) r o the wiergy cliistrr DC.4 < S mi
O R.
6, < 0.3 rad and PT > 30 Gel'
h s s > 100 GeC'
This analysis is only concerneci with high rnüss events. as calculated by e c p t i o n
.5.33. This cut effectively removes low Q2 S C DIS backgrounds.
The control plots 7.4 and 7.5 are made with an additional cut reqiiiring at least one
reconstructed jet with qli < 3.3 and ET,,i > 30 Ge\'. In the data 3346 events were
selected while 3270 events were expected from NC DIS and php backgrounds. The data
shown iri the control plots is in good agreement with espectations. As seen in figure 7.6.
the observed invariant mass spectrum agrees with MC simulation reasonably well up to
about 210 GeV. where there is sornewhat of an escess. These events can be characterized
as single jet events and low y.
7.3.3 Jet Cuts
-4s previously mentioned. inost of thp topologies iirider investigation contain more thiiri
two jets. Having selected a saniple of high mifis S C çxvents. the jct sigrial characteristic
is exploited in order to search for possible signals in tlic data beyorid wvtiat is eqwcted
from the S M processes.
If thc following ctit is satisfied tlien t t i ~ evmt is piit iri t l i ~ P jp ts sample: otlierwisc~.
the event is placecl in the e jet saniplc.
At least two good jets are reqiiirecl.
A gmd jet is defined to tic! one with 11 > 2.5 arid 2it least 10 Gt4- in Er.
- T ~ I < 2.5 and ET,JI >
The highest ET jet lias to pus an optiniized .jet ciit as a fiinction o f tht!
invariant mass of the E jet(s) system. The ctit is illiistrnted in figiirr 7.7. I t
starts at .- 18 Ge\' at 100 Ge\' and reachcs a platmu of - 30 Ge\- abow
200 Gel'. The net affect of this cut is an increase in the acccptance for lowvcr
reconstructed invariant mas. in events mith two or more jets.
- rj12 < 2.5 and ETJ2 > 1.5 Ge\*
The second highest ET jet must h w e signifiant Er.
With multiple jets in the final state the jets ~ o u l d tend to have lowi-er ET than in the
single jet final state. Even if a real squark cascade decay does not pass the above criteria.
Figure 7.4: Distributions u j jet uanubles for the preselccted sumple. .4 boue: the pseudoru-
pidit!) ( q ) and transverse energq (ET) /or th': htyhest ET jet. Beloi~.: thc pst~rdorapidi ty
(7) und transverse eriergg (ET) f u r the second hzqhest ET je t . The p o ~ n t . ~ wlth rrror h r m
are the data. The histogrum is the total expected hhackgro~und (~VC+php) . The backywirrid
histogram is n o n a l i r e d to the dota lurrrinosity.
Figure 7.5: distribution^ of kinemutic variahles /or t he p~eselectrd sarnple. .-Lhove: the
polar angle of the positron candidate (O,) and the ratio of the t r n n s v e r s ~ nromrntrrrrl
(PT,h) to the transuerse energy (Er.h) !or the hudrorric finul stute. Below: the c«rinble
y. calcdated using the double angle rnethod (!/D;L). and the transverse ntonzentwn of the
electron candidate (PT..r). The points with error burs «re the duta. The hrstogmrri i.s
the total expected background (NC+phpj. The background histogram is norrnalized to the
data lurninosity.
Figure 7.6: Aboue: dist7-ibution of loglo(QZ). culculated using the double ande rnethod
for the preselected sampie. Below: the reconstructed muss d i s t r ib~ t ion for the preselected
sample. The points with error bars are the data. The histogram is the total expected
background (NC+php). The background hzstograrn is n o n ~ d i z e d to the data lzrrninosity.
................ r i - a ................. .................... J601-r.o*.o......... a . . . . . . ....................
P L -
240 260
Figure 7.7: The ET distribution of the highest ET j e t us cr filnction O/ the rrruuriant rrrn.ss
of the e + jet(s) system. The curue mdicates the optzrnized jet Epr cul. Eucnts uboue the
cut survive. The inlay depicts the cut /or clanty.
it in- nevertheless pass the e jet sclection. In this nay. cascade decays which do not
pass the e jets selection may survive. thereby increasiiig the overall squark acceptance.
7.3.4 e jet Sample
A squark decaying via 41, will optimally only have a positron anci a jet in the final state.
As a result, the Er of the decay positron and jet will be very iiigh. The ciits u s d for
this s m p l e were:
At least 1 jet with 01 < 2.5 and ETJI > 30 Ge\.
P7 > 30 GeV
go.-\ > ~ ( - \ l < n u )
h scalar particle tias a Hat y distribution as secm in figiirr 7.8. This is an optiniiwd
ciit to further suppress the SC backgrouiid.
These cuts rvere derived from the search for a hi@ n i a s e t j e t leptoqiiark rcsonancr
search at ZECS[83]. The events resultirig in the excess mentioned carlier (sep figurr 7.6)
fa11 into this e jet sample since tliey contain only onc jet. Tlieir !/ distribution. ~ > < ~ i i k t ~ I at
loa y. is not consistent ni th that of the d e c q of a scalar resonancc. rvtiicti is Hat. Tliey
are removed by the final optimized y (see figure 7.8) ctit.
7.3.5 e jets Sample
In this c a t e g o ~ . the cvent contains at least two jets of high Er and a high PT
candidate. The following additional cuts are designed to further promote niultijet events:
Single jet events peak at PT/ET = 1.
- * 9 . Y .
zmoF -] 1 NCDIS : ; Signai MC: e jets final state
17st * Z Signal MC: e jet flnal statr -
h 1 : . . . . - . 0.9 5:': . ' . . . . . . . . . . . . 0.8 i : * :. ..: ......... .. S . . . . . . .
0.7 i : : . : : . . , - . . +. . . . . . . . . . . I... i........ . .
Figure 7.5: (a ) The y distribution for iVC DIS cornpured to a sarnple cascade dccuy (r j r ts 1
and u sample P, decay (e jet) wzth arbitrary nonriulizatzons. ( b ) The optmrzed y (:ut a.s
a function of the rnvarinnt mass. Events aboue the ç z l n ~ survive.
Q;, > 800 GeV2
Since the e= arising from a sccondary clccay of a gaugino is predominantly recoti-
structed in the BCAL or FCAL. the effective Q2 due to the polar angle of the e= is
very large. See figure 7.9. The rnean effective Q2 increases with the rrconstriicted
squark mas.
0.4 < 1~0.4 < 0.95
As illustrated in figure i.8(a). multijet events of high invariant mass peak at high
y. Conversely. the main source of background. high Q2 NC. peaks at low y. so the
lower y acts as a very good discriminant between signal and background. The upper
cut on y is applied to remove photoproduction events where fake e= candidates in
CHAPTER 7. EVENT SELECTION
the very forward region contaminate the sample.
4 %. - &30 - - NC DIS
M , ~ ~ = ~ w GeV
Figure 7.9: The Log,,(Q2) distnbution fo r iVC DIS coinpnred to that of a cascude .squurk
with arbitrarg nonnulzzation. The cut is irtdicnted by t h fine in the figvre.
The restilting data set is finally segmented by the charge of the identifiecl lepton.
Should the charge of the lepton candidate be identifieci as negative. it is classifiecl as an
e-jets event. othenvise it is classified as an c'jets candidate. .-\ track is deemed to be
negative if it should satisfy the following conditions:
Vertes Refitted Track
0 The track must have hits in at least the 1st superlayer and the 3rd superlayer
If a track does not have hits in a t least the first three superlayers. the curvature
and momentum measurements from the CTD are no longer sufficiently accurate[S4].
The longer the track the better several track propertics becorne. For esample. the
momentum resolution improves with the square of the track.
The track charge must be negative at the 30 level
30 e - candidate was selected from the data.
7.3.6 Selection Efficiencies and Squark Mass Resolut ions
The efficiencies for cascade squark cfecays are illiistratccl iri figure 7.10. They range frorri
30% to 60%) from squark masses of 100 Gr\- to 280 Gd' . The m a s resoliition (figure
7.1 1) for the cascade squark dec-s ranges iror~i 3 - 7 Ge\'. slightly largcr tlian thr
&, squark decay resolutions which range froni 2.5 - 4 Gt4*.
7.3.7 Final NC-like Event Samples
The nunibers of observed events in the threc SC-likr diannels wwe in iigrrcnierit with t.hr
espectations froni SS1 backgroiinci and are suniniarizcd in table 7.1. So ovrrall r s w s s
of events is observed and no sign of a mass peak appcars in figures 7-12 iind 7-13. wtiich
show the invariant niass distributions Tor the c jet and e jets reconstriicted clièirincls
respectively. These distributions are an important result. They show that the ptiysirs of
the multijet topologies being considered is well described by the SM.
The selection of CC-iike events is not as dean as SC-like events. There is a lot of
contamination from non-ep processes due to the requirement of an imbalance of energ.
(yT). This background is comprised of cosmic and beam halo induced muons and events
containing sparking calorimeter cells. and has already been discussed in section 4.3.2. The
rrtuss. The range of ef icf ic ie t~ies over the gmer-uterl SL,'Sk' pliiisr spuce points ut errch rnn.s.s
triggcr itself rejects sonie of the more obvious background events. but rejection beconi~s
complicated when these events overlap legitimate ep processes. 111 general. such events
have an extremely localized large energv deposit. usually al1 in a single cell. creating the
7.4.1 Cleaning cuts
This section details cuts which remove non-ep events. These are events arising frorn
beam-gas. halo or cosmic muons or detector hardware problerns. which are sufficiently
like ep events to pass through the trigger and DAQ chain. The following cuts were used:
7 - - - - O ta, 120 IM 160 i i i iw rto wo a i ~ rso 100 IZO 1.10 160 1811 tûû 220 2.10 260 280
BIau ( CeV) %la.%s t CcV 1
Figure 7.1 1: (a ) The mass resolution for cascade squark decu!j.s lnto P jets os n junctlori
of the squark mass. f h ) The muss re.so1ution for the IP, sq~rurk decays ns n furictzorr oj
the squürk mnss. The range O/ resolutiorls o.ver the lenerutrd SUSY phuse spacr at rnch
mass is shown.
0.2 PT < 3
Sparks in cells overlapping a i t h an e p prowss givc n large PT. and woiild I~rcome
backgrounds. Isolated cells with a bad energi- inibalancc are rerriovd frorri the Pr
calculation described in equation 5.23 and the rernaining overall Pr is designatrti
h g o o d .
f i h z g h / y* < 0.8
Sparks can potentially cause a very large fake eriergy. Hence the event is rejected if
the PT measured by the calorimeter ce11 with the highest individual eriergv (PThigh)
is more than 80% of the total PT.
1 Reconstructed Channel 1 .Yob, 1 .V,,, I r
e j e t
Table 7.1 : Svmmary of data sample everit nurnber.7 ccompared to expect ed background frorrr
NC DIS (Ariadne) and php ( H e m i g Resolued and Dir-ect ET > 50 G e ç 7 .
e' jets
e- jets
e+jet Invariant Mass
I 1
Figure 7.12: The reconstmcted ntuss distnbutiori for the ei jet final sample. The pozrits
Wth error bars are the data. The histoyrarri is the tutu1 e q e c t e d buckgro~und ( K t p h p ) .
The background hzstogmrn is normalized to the data hminosity.
70
66
O
S I 3~ 6 ,
X E k 5
0.20 & 0.01
e+jets Invariant Mass
_ -* - L A -- +
ioo i20 140 ' 160 180 200 220 240 - 260 Mass(GeV)
Figure 7.13: The reconstmcted rnass distdution /or the e+ jets &na[ sample. The points
with error bars are the data. The histoyrum is the total eqected background (iVC+php).
The hackground histograrn is norntalized to the datu luirrinositg.
a HAC fraction cuts
The particles emerging [rom genuine ep collisions in the interaction region miist
hit the EklC section beforc the H X . so one rvould not erpect these events to be
completely hadronic. The folloning cuts reject cosmic and halo muons which coulti
miss the EMC sections of the CAL.
This variable is defined as the ciifference betwcri the azimuthal ariglr of t h e as
measured by the CAL and bu the CTD. This rrcpirenicnt further rdiices cosniir
and halo muon backgrounds.
- f i > 20 Gel* and 19 < 2 radians
OR
< 20 GeC' and 14 < 1 radians
Cosmic and halo nliion identification
Pattern matching algorithms M u T R I G [ ~ ~ ] . ISITAMU. ahalo and comcos (8.51 are iised
to further suppress muon related non-rp backgroiinds.
Timing cuts
Non-ep backgrounds may give a timing measurement which is inconsistent with an
ep collision. The cuts performed a t the TLT and described in equation -4.10 are
repeated and tightened. See appendix A. 1 for details.
7.4.2 Selection cuts
Analogous to the preselection performed in the NC-like case. this section outlincs ttic first
step in selecting a CC sample with high Q'. For CC events. well reconstructed events
with P)7 are preferentially chosen. Tracking and electron/positron firicling. in ortler to
remove any NC events. are important in the followiiig event selection:
DST (34 or 35 or 36) and 37
Along mith the high ET or high isllznd ET trigger selections whicti were comnion t,o
the YC-like selection. the CC DST bit 34 is also rccpired. htlditionally. DST l i t
37. which dernancis a reconstructd or TLT v e r t a o f -GO cm < Z,:,, < 120 (:m. is
reqiiired.
Vertex Requirement - -- - -- -
.-\ tracking vertex is requireti. idiich rriust satisfy Z,,,/ < 50 cm.
SC Re-jection
If a scattered e= is identified (em prohabi l i tp 0.001) in the calorinietrr thmi thrl
event is classified as a SC background and rcjectecl. See appendis A.3.
At least one good vertex track (Ngtrk)
A good vertex track is one fitted to the vertes and has a f i > 0.2 Ge\. in the
angular range of 15" < 0 < 165". the CTD ücceptance. Derriünding a good track
reduces non-ep backgrounds.
Ngtrk > O.X(Ntrk - 20)
Ngtrk is the number of good vertes tracks. defined above. and St rk is the total
number of reconstructed tracks. Beam-gas events typically have many tracks but
only a few of them are assigned to the primary vertes. This cut effectively removcs
them. Figure 7.14 shows events in the (numher of good tracks)-(number of al1
tracks) plane, excluding the cut for al1 final candidates with at least one good jet.
A ( - l i r ) > 12 Ge\-
f i ( - l i r ) iç the net missing transverse momentuni calculated from al1 cells in the
calorimeter eacluding the first ring of FC.-\L cells immediately siirrounding the
beam hole. This f i ( - lir) ciistribution is shown in figure Ï . l j ( a ) . cscluding the
cut for al1 final candidates with at lcast otie good jet.
& > 15 GeC*
Significant net transverse nionientiirn is the priniary signature of a CC-like pvrnt .
This cut was optimized for the u jets topolugirs as ;i function of t h r sqiiark niass.
The optimal cut is slightly lowr at squiirk riiassc~s below 1'20 Gr\'. A lorver ciit
increases contamination frorri biickgrou~id. tliereforc tliis global (:ut was üssiimrd for
al1 masses and topologies. Figure ;.lEi( b ) shows t hr 47 clist ribiition before applying
the cut for al1 final candidates with nt least one good jet. The f i distribution in
figure 7-16 shows good agreement betwcn data and MC background.
These cleaning and selectiori cuts for the CC DIS sarriple and their effect on the data
sarnple are summarized in table 7.2.
The ZECS high Q2 CC analysis[66] applies al1 thesc ciits plus the kinematic ciits
Q' > 200 GeV2 and y < 0.9. By performing these cuts the same sample of events is
selected and agreement between the data and SIC background is obsened. as seen in
figures 7.16 and 7.17. This data sample contains 857 events. to be cornpared mith 880.7
events expected from CC DIS and photoproduction backgrounds.
/ ( non-ep: halo p. cosrnic 11 1
Selection Cut
HAC fraction
Il 1
NC Cut I
1 89999 1 92.1 n
Events Selected
Table 7.2: Sumrnaq of the cleuniny and selection cuts /or. the CC DIS samplti. The CC
CC Efficiency 'Z
Eficzency is n o m a l i z e d to the DST tngger sefection.
100
100
DST Trigger
CAL Timing
46602s
35-4922
' O 20 40 60 80 100 Ntrk Ntrk
Figure 7.14: D i s t ~ b ~ ~ t i ~ n . ~ of euerits in the (nirmbcr of good trucks)-(rrurnher of (111 truck.^)
plane for (a) Data and ( b ) CC DIS MC simulutioris.
7.4.3 Signal Enhancement
Once a sarnple of high Q2 CC events is isoleted. one can take advantage of the high mas.
high y. multijet cliaracteristics of the t o p o l o ~ t hat are b ~ i n g investigated. To ~ n h a n c r
the acceptance in the high Q' CC DIS sarrq.de rcsiilting from thc cleaning and selectiori
cuts. the following cuts rvere applied:
Mass > 100 Ge\-
As in the NC-like cases a high invariant mass of the u jets sytem. as calculated by
equation 5.50. is required.
y > 0.4
This cut is an effective cut on the E - Pz of the entire event. A high y cut selects
multijet events. as they are more likely to be spread out throughout the detector
Figure 7.13: The (a) PT(- l i r ) und ( b ) PT distributions before the applzcatiun o,f their se-
lection. The points are the data und the I~istoyram is the eqec ta t iun /rom MC szmulations
normalzzed to the data luminosity. The cuts are i~idicated hy the lines.
Num good jets Y
Pt (GeV) O 50 100 150 ?DO
Et (GeV)
Figure 7.16: -4bove: the nurnber of good jets and y dist7-ib-utioirs after preselectiori. Beloru:
the PT and Er d i s tnbu t ion~ after preselection. The points rrpresent the datu und t h e
histogram represents the SM ex-pectution (CC+phpj normalzzed to the data luminusit?y.
Mass (GeV)
Figure 7.1 7: The log,, (Q') und in-uanant muss aber preselection.
each contributing to a higher E - Pz rneasurement and hence a higher y.
0 Number of good jets 2 2
Again as in the XC-like multijet topologies. squark decays with a u in the final
state are high y events with two or more good jets.
The effects of the cuts are displayed in figure 7.1S(ii) for the gencratcd SCSY pliiisc
space points. Note that the range of efficiencies for rlir lowest tmo sqiiark niass bins arc
noticeably smaller than the range for the corresporiding squark masses above 120 G d ' .
This is the result of two effects. First. the PT riit dorriiriates the diciency at the lowrst
m a s bins. Second. a t lower squark niitsscs. fcww topologies esist. as a consi.clurriw of
t h assurnption that the LSP is tlic < y . Hmcr. if .\lit;. wtiich is cleterniinecl t ~ y thr choicr
of the SL'SY parameters. is greatcr ttinri thi. squark ninss. t.heri thet phase spacr point
will only contribute to the efficieiirit~s a t q u a r k niiissvs ; h o w the riiass of the LSP.
Typical values of the efficiency for sclerting sqiiarks range from 4 35% at .Wq <
120 Gel- to - 50% at .LI4 > 120 Gt-i-. Thr triii.ss resolutioii for cascadr ~(lliiiïk cicr*+-s
with a neutrino in the final state is roiighly constant at 12 Gel' as a function of ttir
squark m a s (figure T.lS(b)).
There are 33 events rcmaining after iill cuts. in agreement with the SM ~spectat ior i
- from .\lC background as summarized iri table 1.3. Figure 7.19 shows the iiivariant mass
spectrum of the selected events conipared ivith ttitb MC prediction. As for SC-like ewnts .
there is no evidence of a squark signal. and the data is well describeci bu the SM.
Figure 7.18: (a) The eficiencies urid ( 6 ) resolutioil.~ / o r . (:uscride squnrk i iemjs into o . j r ts
us a function of the q u a r k muss. The range of efi:8iciencw.s und rtxol~~tzoris ourr the yeri-
erated SUSY phase space points at each muss is sho,wrc. The mass points helnu~ 140 C d *
are domznated b g the the PT cut.
Table 7.3: Summary of the number of events in the CC-iike data sample cornpared to
1 I Reconst riicted Channel 1 .Yah.$ i
expected background /rom CC DIS (.hiadne) und php ( H e m z g Resolved and Direct ET >
50 GeV).
;36 k 3 A
ujets 35
Mass (GeV)
Figure 7.19: The reconstructed rnass distribution for the v jets final nample. The points
wzth error bars are the data. T h e histograrn r s the total clrpected background (!VC+php).
The background histogram is nonnalired to the data furninosit~.
Chapter 8
Limit Setting Procedure
For an integrated luminosity. C. t h r cspwtcd differeiitial niass distribution in an;ilysis
channel c is the surn of the background. b,(.\I). iirid t h sigrial. s,(.\I: O). whcrt. t h r;ittl
of signal events is proportional to the sqiiark prodiictioti cross section. 0. The prohahilit-
density in the rnass for t tic events in a niass iriterval [.\ I l ' , . .llhIqh] is given by
The functions h,(.\l) w r e tletrrniirird ty fi ts to t h . iwkgroiiiid Ilontc. Carlo. Tho
.%iilh total expected background for channel r* is defincd as B, = J\I,ou, b,(.\l)dM. The mass
distribution for the signal. s,(.\l: a) . is taken to be a Gaussian with a root meari sqiiarp
( R M ) width which has been determined by fits to the signal Slonte Carlo sarnples. The
rnass window [ M l . . -Crhrgh] is taken to be a f 3 0 wiridow centered on JI4. The nornial-
.'fhi9h ization of s,(m: o) is given by CE,^ = J.lC\Lou, . s r ( JI: 0)dSL. n-ticre F , is the probatdit?-
that a squark ivill be accepted in channel c with .\4, < JI < .\Ihigh. It is cornputeci
as a sum over the decay modes. d, of the product of the branching ratio. &. and the
cross efficiency. cf. which is the probability for a squark which decays via mode d to be
accepted in channel c: - v d t E ~ y s
C, = C J~c:. d= 1
The likelihood to observe Kbs events in charinel c in the mass interval [.\ll,. .\.fhLgh] is
written as a product of the Poisson probability to observe .yb" events and the probability
densities for the reconstructed masses ,El, of each observed event:
The likelihood for al1 channels is then givcii by
The 95% confidence limit (CL) on the cross swtioii is obtained hy solvirig
The upper lirnit on the coupling A' is calrulatod usirig the relation
where o,vrv,l is the cross section calculateci using the riiirrow widt h approsimation (NU )
and hl,,,, is the coiipling used in the c~lculation of thr o .~~ i . . -~ . Siricr the hrancfiing ratios
are functions of the coupling A'. the Likelihoods arc calculateci iterativcly givcn an initial
guess for S.
8.1 Inclusion of systematic uncert aint ies
A systematic error can in incorporated in the limit procedure by convoluting a Gaussian
with the Poisson probability and solving equation 8.5:
where p, = Leo, = B, n = XobS From equation 8.3 and r; is the fractional systetnatic
uncertainty. The double integration over *y and p, (or alteriiatively o) is achieved by
performing a single integration dong the contour which ~r~c~i r r i izes the ititegrand in 8.1
witli respect to y for a given p,[86]. The valiies of ; for tvhich this is triie at a giwn 11,
are the roots of the following equation:
8.2 Interpolation and limits in the SUSY phase space
Limits on 0 and A' are derived at points «il a grid of t h SCSY paramtwrs ( p . .\L) w t i ~ r r
-200 < p < 200 ( in 20 Ge\' intervals) and SO c .\& < 310 ( i r i 10 Gt.1- int~rvals). .At
each grid point the limits are clerived as a [iinctioti of .\Id frorn 100 to 250 G d * in 1 Gc4'
intervals.
The masses of the gauginos are cletcrniirird by t hc SCSY parariietcrs. Monte Carlo
data sets are generated as descrihwi in srctiori 6.2 aritl not at each poitit of thcl l i r r i i t
grici. Eiglit phase space points were srle(:tcd siicti t h t thry mronipass thc niiLss apiicc
of the My:. .\f,o and Mt=. The conjecture is rtiacle that the rfficiericies. c,. and the cross ? 1
efficiencies. É:. depend on the masses of the gauginos. Hence in order to determine the
efficiencies at a phase space point which h a not been generatetl. a weighted linear comhi-
nation of the generated point efficiencies is calculateci. The weights. tc,. are proportional
to the square of the difference in gaugino niasses bctrvecri tlir desired and generated
SLSY phase space point such that
The weights are oormalized such that 12;' ,i = 1. Hence. the probability for a squark
which decays via mode d to be accepted in channel c given in 8.2 is non written
.L;lccays + C l p d p
(S. IO)
8.3 S ystemat ic Uncert aint ies
The systernatic uncertainty considered in the derivation of the upper limit for & sqiiark
production is estimated to be 13%. Systematic errors c m affect factors in cquation 5.3
such as the luminosity. the background distribution or scl~ctiori rfficiency. T h r following
sources of systernatic error werc cunsirlercid:
0 Luminosity measurement
The systematic uncertainty on the p - p data saniple collectixl during ttw years 1994-
1997 is estimated to be 1.6%[87].
O Electron/Positron finding
An estimate of the em findirig cfficieti(ry is rstracted by rotriparing em witli thr
S INISTRA neural net elect romagnrt ic cliister fincler. The variation betwceri the tiw
e' finders is found to be less than .j%,[,I;O].
Calorimeter Energy scale
According to stiidies(88. 89. 901. the absoiilte energy scalc of the calorimeter is
unknoan to 13%. This translates into a riiliximuni change of 1.7% in the select ion
efficiency and is included in figure 8. 1.
0 Vertex
The vertex requirement was varied by I l 5 cm. which changed the squark selection
efficiency by 2% and is included in figure 8.1.
0 SeIection cuts
.Al1 cuts based on CAL eriergies. E.. ET. f i . jet ET. 5 and invariant m u s were
varied by the uncertainty of energ. scale (3%) indiviclually. The effect was at niost.
2% in the signal selection. See figure 8.1.
0 Interpolation
The interpolation of the limits in the SCSY phase spacc clescribetl in section 8.2 was
evaluüted by using another weight. Instcaci of t h e wcight ansatz usecl in eqiiatiori
8.9 the following was used:
The effect ori the liniit was at rtiost 1%.
0 Background estimation
The effect of an error on the hackgroiirid t>stitiiirt,e uscd iri the iipper limit is rliffic:tilt
to assess as it depends also oii the niiitiber o f ot)svrvetl tivritits. A siriiplt~ estiriiiitt.
can he obtained by the differcnce bettvcen two SIC background samples. Tlic
background MC SIEPS ivas iised as an alternatiw SIC. Tlie ~ffect \vas determinrd
to be 6.4%.
0 Theoretical uncertainty.
The parton density uncertainty should be takeri into account. Ckianging the par-
ton density function (PDF) used in the cross section calculation resulted in an
uncertainty of 9.4%.
Table 8.1 sunimarizes the sources of systematic uncertainty taken into account. The
various systematic errors are assumed EO be uncorrelated and are added in quadrature
to give the total systeniatic error of 13%.
Lumiriosity
e' findirig
CAL energ? sralr
Vert es
Selectiori Ciits
Interpolation
Background estirriatiori
Table 8.1: Sumnrun~ of syst emattc iir~crrturntces corisidernl.
1 TOTAL 1 3
Chapter 9
Results
The search for PC, supersymrnetric particles was pdorriiecl iising the 47.7 of r'p
data collected between 1994 to 1997 at a cc3ntrtb of riiilss ciiergy of 300 Ge\'. Sqiiarks
produced in e l quark fusion. via the 61, coiipling A;, , . cari (lrcay to r-quark o r gaugino-
quark via a supersymmetric gauge clemy. rpsulting in a final state with niiiltiplc j m arid
a lepton that could be a positron. electrori or neutrino.
There !vas agreement between t hc tiat a and t hr csptx: t at ion frorn the SM backgro tinds
consiciered in al1 the reconst rticted cliarin~ls. Thtl signal i4Ficieticy for any topology o r
SLSY phase space point ranges froni - 30% - 60%. In the absencc of any signal. limits
are set on the & coupling A;,, as a function of the sqiiark mass a ~ i d the SUSY paranietcrs
(AI?. p. tan 3). .A systematic error of 13% aas incluclcd in the determination of the liniit
as outlined in section 8.1.
Given a squark mass one can deriw an upper limit on the coupling Xij,. To obtain these
limits one combines the different squark decay channels. Therefore. it is necessa- to
know the branching ratios for each channel. These depend on the mass of the sqiiark.
the coupling Xi,, and also on the SCSY parameters. Recall that the phenonienology
depends on the nature of the LSP. The possible channels are the same whether the LSP
is 7 dorninated or 2 dorninated. but the branching ratios are nevertheless different.
The search was performed at points on a gricl in (-14. p. .&) in the region 80 5 JI2 5
310 GeC'. -200 5 p 5 200 Ge\* and 100 5 .Il4 < 230 Ge\*. The gricl spacings w r r
10 GeV. 20 GeV and 1 Ge\- for A&. p and .\fi respcctively. Two values of tan .i wcre
considered. 2 and 10. Limits aere orily calculated at points where thc LSP is tlic <y w i t h
a mass exceeding 40 GeV.
The erpected limits follow the actiial 95% CL lir~iits as secci in figure 9.1. whicli is
another indication that there is rio significarit t3scrss of data over the SIC expcctatioii in
ttic range of masses concerneci.
The limits do not var- çreatly over the SCSY pli;~sil space. Huw~ver. tlir liniits arr
somewhat better when the is lieavier (figiirc 9.2) . Recirll from figiirr 3.3 tha t .Uiy
scales mith JI2.
One other point to note is that t h r branching ratio to eT jets is larger tlian to e- j r r s
because the f; can only decay into an P - . Apperidis B shows the brarichirig ratios and
efficiency curves for various phase spact. points.
The limits a t large squark mass rend to converge to the same liniit as illiistrated hy
figure 9.3. This esplains that at large mus w~ are riot sensitive and set large iipper lirnirs
on A;,,. At large couplings. the cross section for eq + (i + eq. is not depindent on the
gaugino masses and hence the SCSY pliase space.
Xo strong variation in the limits with 16. p . and tan J = 2.10 aas observed. The
95% CL upper limits on the coupling A;,, ranged froni 0.01 for squark masses of 100 Ge\*
to about 0.8 for squark masses of 280 Gel-. If we select a X i J I = 0.3 then we can rule
out squarks below a mass of 251 - 262 GeV depending on position in the SCSY phase
J Ili -
Figure 9.1: A;,, limits as a /unclion O/ .LIq /or f i e d -14 = 290 GeV. I L = -200.200 Gei'
and tan 3 = 2 . The thick cunie represents the uctual limit and the thin curue indzcatrs
the ezpected limit.
Figure 9.2: The 95% CL upper lirrrzt on A;, , rn thrl SCSY pornmeler (.\-. I L 1 s p u w for
.\lj = 200 Ge\'. (a ) tan .j = 2. ( b ) tan 3 = 10. The scule on the range indzccttr.~ thr ridur
of A',,, . Recall that the mass f~incwa.ses with ~rrcrrri.srrig .\-. .4 lirnxt w(ts not .wt rn the
central white region. il region o / p l ~ u s e spaw whfrre thr LSP is the lightest chargino.
space (figure 9.3).
9.2 Ot her experiments
While o t her HEP erperiments Iiaw performed S CSY particle searclies. HERA reniains
one of the rnost cornpetitive n i th respect to the A:,, & coupling.
Extensive 41SShI searches with R-paritu conservation have been performed iit HERA.
LEP and the Tevatron. al1 of which resultcd in the absense of a signal. R-ithin ZECS. a
search for selectron-squark production via the diagram given in figure 9.4 aas performed
at ZECS. That analysis excluded (rn; + m,#? < 77 GeV at 95% CL for mil = 40 Ge\'
Unconstrained MSSM
- , - . - Neutrinoless
Figure 9.3: 95% CL ezcfrisron upprr hrrt~ts orr tlw P, coupling as a ji~rictron of t he
squark mass. For each squark rnass. a scan O/ the SCTSY parumeter space (1b. p ) huas
been perfonned wzth the ma~inium (mrnzrnurrr) value O/ the coupiing lirnit represerited
by the upper (lower) curves. The solrd (dashed) line tndzcates the pliuse spuce scan
wzth tan 3 = 2 (10). .Usa piotted am the limits on A;, , !rom neutrinole.ss 33 decug
e q e n m e n t s und the lirnzts on A;,, /mm .Ltoniac Punty Violation experirnents. Regzons
aboue the curues are excluded.
Figure 9.4: Dingrnrn for the ndection ~pmrk. production process (11 HER.4.
and large .\ISSII paranieter Ip( v;tliii~s[I)l]. H o w w r . tlicse rcstilrs. ils n-itli ariy d i v r
SISS'VI search wit h strict R-pari ty coristnat ion. do tiot apply to the rt?sults pr~senrrd in
this thesis.
Both ZECS[92] and H1[93] have perfornied lepton fiavour violation srerrhes. t J q ->
pq. rq. Tliese searches whose results cün be interpretrcl in terms of @, couplings A!,,, and
A\,, and limits are gerierally set. on the prodiict of t h two couplings. Tticse coiiplings
do not fa11 into the scope of topologirs coverctl iii tliis thesis.
The LEP experiments influenccd the consideratioti to set limits only rvliere the 31,? >
40 Ge\' due to their constraints on the SCSY parartictcr sprice in their gaiigino searchesi941.
The result obtained by this analysis is compared to the most stringent indirect limits
in figure 9.3. The production of a ü squark via a A;,, coupling is severel- constrainetl bu
the non-observation of neutrinoless double beta decay[95]. Atomic Pari ty Violat ion[OG]
(APV) experiments set limits on L: (as well as i!) but are only better than the ZECS
limits at the very high masses. Otherwise HERA provides the best limits on A;,,. where
j = 2.3. The D0[97] and CDF[98] esperirnents at tlie Tevatron have performed searclies
for scalar leptoquarks which effectirely rule out ü', squark masses below 205 Gel ' as long
as the branching ratio 6; + eçq is greater than 50%. Since tliis P, branching ratio
can be quite small. these constraints become weaker as the branching ratio via direct
& decreases.
The only true cornpetitor for the ZEUS results corne frorn ttir other collider esperi-
ment at H E R L H l . The H 1 analysis[99] ha roiighly the samc sensitirity and rfficieiicy
as this analysis and hence their resiilts are comparable to those froni this analysis.
Al1 the searches discussed abow liave prover) to bo a Fiirther confirniat,ioti of th. SU.
Yet SCSY remains to be the favourrtl theory for ptiysics bcyorid the S5t by tlieorists. Thr
clifficulty in presenting limits from SCSY searclies is tliat SCSY lias a rniilticlini~risional
phasespace but results can only bt. prcwritd iri iit riiost three dimensions. Ttic rcsiilts
given in figure 9.3 show the tmo variables. A;,, irnti squark rnass. wliich HERA can sct
the most stringent limits.
Chapter 10
Conclusion
-1 search for R-parity violatirigsupersyriitiit~tric piirtic4t.s has been perforniecl iising 47.7 ph-'
of e - p data taken using the ZEUS (imvxor iit LIER;\ froni 1994 to 1997. So wiclciicc~
\vas discovered for squarks. which are procliict!tl via an & couplirig A',,,. decayiiig into
e jet. e jets or u jets. Cpper limits at 9.5% CL oii the coupling A;,, as a hinction of the
squark mass frorn 100 - 280 Gd' arr derivrcl. for points in the SCSY parairieter spwr
(JG. p. tan 3). This analysis constitutes the tirst sucli search for ZEUS.
The limits were found iiot to vary grwtly nwr tlir SLSY pararritlt,cr spacr im(l t g i c i t l l ~
range from 0.01 at a squark n i a s of 100 Ge\- to 0.8 at 280 Ge\'. At a fisctl squark niiLss
an iipper limit on the coupling strength can he es t rx t ed or conversely at a given coiipling
strength. one can exclude squark niasses below a certain limit. inferred froni figure 9.3.
For example. at a coupling strength on the orcler of the electromagnetic scale. A;,, = 0.3.
squarks below 254 GeV are excludeci over the entire SLSY paranieter space considerecl.
The invariant n i a s distributions in figures 7.12. 7.13 and 7.19 show ttiüt the SN
explains the specific high mass rnultijet topologies considered iri the contest of t his t hesis.
HERA has a unique ability to directly search for couplings L , Q , P ~ due to the lepton
and quark flavours in the initial state. In 1998 and 1999. H E U operated with e - rather
than ef . The analysis here forms a template for the search to be made in this data
set. which opens up HERA's seiisitivity to d i sqiiark production. Moreover. the proton
beam energy was increased in 1998 to 920 Ge\*. corresponding to the larger centre of
mass energ-y of 318 GeV. Since the &, scparks are produced via the s-channel right iip
to the kinematicai Iimit. the new data set will i~icrec~se the i~iass reach of this search.
The HERA facility was being upgraded to increase Iiiminosity a t the tirne this thesis
was being written. With a five-fold increase in HERXs luminosity eupectcd for the post
lumi-upgracle of t hr niachine. the poterit ial for discowry tvill be miicli greatw.
Appendix A
CC-like Cleaning Cuts
A. 1 Calorimeter Timing cuts
The following timing cuts were applied t o hc cwisistcrit with ep collisions:
Xy is the number of PSITs which are uscd in the tinie measurement for the region .Y.
A.2 NC positron/electron criteria
The candidate DIS e' had to satisf!, the stürid;ird NC DIS e' criteria (100i rvith ari
energy greater then -I Ge\- and fi > 30 Ge\-. Otlirr ctits w r e as follows:
El*, < 5 Gel-
The candidate hacl to be t v d l isolateci. T l i ~ t!ricrgy not assigned to th<. t.lcc*troni;ig-
netic cluster within a cone of R = 0.8 in ( 1 1 . (1)-spacc ccntrcd irroiinci thr r.' haci t o
be Iess than 5 Gd*.
If found in the FChL a large elwtron E+r is rrquirecl.
15' < de < 16-1" + P;,/Ee > 0.25
If found within the CTD scceptaiice. a rriatching track (with a DCA of 8 (Sm) is
required with the momentuni as rneasuretl bu the CTD (Pck) to be consistent with
the energy measured by tlir CAL.
RChL candidates are required to have a minimum ET.
Appendix B
Branching Ratios and Signal
Efficiencies
The branctiing ratios and signal efh:ii!ticits for al1 ttw quark clecays rorisiclerecl. arp
presented here for a few representatiw ptiascb s p m l points. The hranching ratios ancl
efficiencics are plotted as a furictiori of t h quark n i a s aiid are evaiiiatetl at t he
limit for that squark mass and phase spacc point.
APPENDIX B. BRANCHING RATIOS AND SIGNAL EFFICIENCIES
III 11, ---+ ---- 1 1 1 ) ~ ( M I I ~ M I ? i n ~ IIMI r n )
1 0 -
111
III
Figure B.l: Branching rutios r r s u function of squurk rntrss in Gr V.
Io r 111
:Y- .,f X . 2 ' " "CC
Figure B.?: Branchiny ratios as a function of squark mass in Ge C:
Figure 8.3: Brunching rutios o s a fmc t ion of squark rrcuss in Gr C'
-2 : lu .,FI. p q c c II)
II~) an
1 0
Ili
111
Figure B.4: Branchzng ratios as a Junction of squark mass in Ge V.
Figure B.5: Ef i c i enc ;~~ us u /unction O/ sqtrark mass in Gr? Ce.
APPENDIX B. BRANCHING RATIOS AND SIGNAL EFFICIENCIES
-r c C! -. C' e- ; . . r C!-. . . Cm: -;:21
1 C F - - - - - .I--- "' 3 .l : c 1 ' r r r r O .: :
*: Io . -
r K 1 p.- ...
L r - - - - . -:. - -__ --- i - - . __-- LI I IMI 31i
w - - - 7 - - ' -
.I - 'W - -
II) 2 ' S C .J :
III ----a -- -+- - -- --.-----
I I
111 1
II) J
II)
Figure B.6: E ' c i e n c y as ( r junction u,f quark muss in Ge CF.
Appendix C
The ZEUS Collaboration
J . Breitweg, S. Chehnov. .II. Derrick. D. tirakauer. S. hlagill. B. Musgrave. A. Prllegririo.
J . Repond. R. Stanek. R. l'oshida
..lr-]onne National Laboratory. .4 rgonrrr. IL . ITS.4
b1.C.K. Mat tingly
.4ndre,ws University. Berrien Sprinqs. MI, I3.4
P. Antonioli. G. Bari. SI. Basile. L. Bellagamba. D. BoscheriniL. A. Bruni. G. Briirii.
G. Cara Romeo. L. Cifarelli'. F. Cinciolo. -4. Contin. hl. Corradi. S. De Püqualti.
P. Giusti. G. Iacobucci. G. Levi. A . Slargotti. T. Massam. R. Sania. F. Palmoilari.
-4. Pesci. G. Sartorelli. -4. Zichichi
University and INFN Boloynu. Bologna. Italg f
C. hmelung3. A. Bornheim4. 1. Brock. K. Coboken". .J. Crittenden. R. DefFlierG. H. Hart-
mann, K. Heinloth7, E. Hilger. P. Irrgang. H.-P. .Mol>. .-\. Kappes8. U.F. Katz. R. Iierger.
E. Paul. .J. Rautenberg.
H. Schnurbusch. A. Stifutkin. J . Tancller. K.C. Voss. A. Weber. H. \Vieber
Phpikalisches Institut der lini.uersitat Bonn. Bonn. Germany
D.S. Bailey. O. Barret. N.H. Brook" B. F o s t d G.P. Heath. H.F. Heath. E. R ~ t l r i ~ u e s ~ ~ .
.J. Scott. R. J . Tapper
H. H. Wills Phgsics Laborutonj, Urriuersit,y (1 f Bristol. Bristol. U. K. "
hI. Capua. .\. 'vlastroberardino. SI. Schioppii. Ci. Siisiiino
Calabria Uniuersit2/. Ph?/iiics Dept-and LVFiV. Cownzti. Itulp f
H.Y. .Jeoung. .].Y. Kim. J.H. Lce. I.T. Lirii. K.J. LIit. '\[.Y. Pacl'
Chonnam National Uniuersit?j. Kiunnyju. Korm "
A. Caldwell. . Liu. S. Liu. B. '\lclla(lo. S. Pagariis. S. Sampson. W.B. Srhniitlkr.
F. Sciulli
Columbia University. Nevis Labs.. Irvingtorr on Hudson. IV. Y.. USA f1
.J. Chwastowski. .A. Eskreys. .I. Figiel. K. Klimck. 1';. Olkicwicz. K. Piotrzkowski? .\[.B. Przy-
bycieri. P. Stopa. L. Zawiejski
Inst. of Nuclear Physics. Craco,w. Poland
B. Bednarek. K. Jelen. D. Kisielelvska. A.'\I. Kowal. T. Kowalski. 'vl. Przybycien. E. Riilikowska-
Zarqbska. L. Suszycki. D. Szuba
Faculty of Physics and iVuclecrr Techniques. dcadern!j O/ iCIining and h ie ta lhry j . Cracow.
Poland J
A. Kotanski
.Jagellonian Uniu., Dept. of Phgsics. Cracoru. Polcind
LAT. Bauerdick. L. Behrens. .1.11. Bietilein. K. Borras. Y. Chiochia. D. Dannheirn.
K. Desler. G. Drews. A. Fox-Slurphy. C. Fricke. F. Goebel. S. Goers. P. Gottlichcr.
R. Graciani. T. Haas. W. Hain. G.F. Hartner. D. Hüsell". K. Hebbel. S. Hillert. SI. Kaseriiannl'.
W. Koch". C. Kotz. H. Kowalski. H. Lalws. L. Liridemannl'. B. Liihr. R. Slirnkel.
J . Slartens. SI. SIartinez. SI. Slilitt:. SI. Sluritz. D. Sotz. S I C Pctrucci. -4. Polirii.
.II. Rohdei. A..-\. Savin. C. Schneekloth. F. Selorib. SI. Sicvers'? S. Stonjek. G. \Wf.
C. Wollmer. C. lbungman. W. Zeuner
Deutsches Elektronen-Synchrotron DESK H«mburg. Genriany
C. Coldewey. A. Lopez-Duran Viarii. A. . \ I t y i b r . S. S(:tilonsteclt. P. B. Srraiil)
DES Y Zeuthen. Zeuthe~i . G e m r w ~ ! ~
G. Barbagli. E. Gallo. A. Parenti. P. Ci . Pelkr
Uniuersit?, and INFN. Florence. i t ( r l ;y f
A. Barnberger. A. Benen. S. Coppola. S. Eisenliardt". P. Markun. H. Raacti. S. \\*GlHr
Fnkultat fiir Physik der Uniuersitüt Frtzburg i. Br.. Freiburg i. Br.. Germanjy '
P.J. Bussey. .\I. Bell. -4.T. Doyle. C. Glasniiinl'. S.\\*. Lee. -4. Liipi. S. SIa~tloniilcl.
G..J. McCance. D.H. S a o n .
L.E. Sinclair. 1.0. Skillicorn. R. Waugh
Dept. of Physics und ..lstronom y, h i u e r s i t y O/ Glasgow. Glasgow. li. K.
1. Bohnet. N. Gendner. C. Holm. A. Neyer-Larsen. H. Salehi. K. Wick
Hamburg University. 1. Institute of E q . Physics. Humburgo Germamg "
APPENDEX C. THE ZEUS COLLABORATION 1-42
T. Carli. A. Garfagnini. 1. Gialaslg. L.K. Gla~lilin'~. D. KÇiraH . R. Klanner. E. Lohrmarin
Harnbvrg Llnive~ssity, II. Institute of Exp. Phgsics. Harnhurg. Germany
R. GonçaloLo, K.R. Long. D.B. hIiller. A D . Tapper. R. Rvalker
Imperia1 College London. High Energy JTuclear Physics Group. London. U.K. "
t-. blallik
University of Iowa. Phpics and Astrurrotny Dept.. IUWU City, USA
P. Cloth, D. Filges
Forschungszentn~m .]dich. Institut jiir Keniph!gs~L. .Jiilich. Gerrnuny
T. Ishii. SI. Kuze. K. Nagano. K. Tokiishiikii". S. Yuriada. Y . 'Ikniazaki
Institute o j Particle and N d e a r Stur1ie.s. KEK. Tsrrk~~bri . .lapan
S.H. Ahn. S.%. Lee. S.K. Park
Korea University. Seoul. Korea
H. Lim. I.H. Park. D. Son
Kyrngpook National Univeruit y. T u e p . K o n x "
F. Barreiro. G. Garcia. O. Gonzalez. L. Labarga. .1. clcl Peso. I. Redonclo'? ..I. Trrr6ri.
M. Vazquez
Univer. Autonoma Madrid. Depto de Fisica Tecincu. ikladnd. Spnzn ''
M. Barbi. F. Corriveau. D.S. Hanna. A. Ochs. S. Padhi. D G . Stairs. SI. LYiiig
iIlcGill University, Dept. of Physics. Moritréul. Québec. Canada ".
T . Tsurugai
Meiji Gakuin University. Faculty O/ General Edttcation. Yokohama. .lapan
APPENDIX C. THE ZEUS COLLABORATION 143
-4. Antonov. V. B a s h k i r o ~ ~ ~ . .LI. Danilov. B.A. Dolgoshein. D. Gladkov. \'. Sosnovtsev.
S. Suchkov
Moscow Engineering Ph pics Institute. hiosco W . Russiu '
R.K. Dementiev. P.F. Ermolov. Eu.A. Golirbkov. 1.1. Katkov. L A . Khein. S.A. lio-
rot kova.
I.A. Korzhavina. V..4. Kuzmin. O.\u. Lukitii\. .-\.S. Proskuryakov. L.11. Shcheglova.
.LX. Solomin. N.N. Vlasov. S.A. Zotkin
Moscow State Universitg. Institute of Yuclear Phgsics. Moscoiu. Russia "
C. Bokel. 51. Botje. 5. Brümnier. .J. Erigelrri. S. Ckijpink. E. Koffcinan. P. K.;ooijnian.
S. Schagen. A. van Sighern. E. Tassi. H. Titlckc. S. Timing. .J..J. Wtliiiis. . J . \i)ssr~brld.
L. Wiggers. E. de Wolf
NIKHEF and Urriversit y of .-Imsterrluirr. .4 nisterdurrr, :Vdherlarid.s '
B. Bylsnia. L.S. Durkin. .J. Gilmorc. C.!L Girisburg. C.L. Kim. T.Y. Ling
Ohio Stnte Universitg. Physics Deprtnrcnt. Colurnbi~s. Ohio. CrS.4
S. Boogert. A.11. Cooper-Sarkar. R.C.E. Deveriish. J . GroBe-t<nettei2". T. htsi is l i i ta .
O. Ruske.
1I.R. Sutton. R. Walczak
Departnient O/ Physics. Unzversity O/ Oxford. Oxjurd U. K. O
A. Bertolin. R. Brugnera. R. Carlin. F. Da1 Corso. C. Dosselli. S. Dusini. S. Limentani.
11. Morandin. 11. Posocco. L. Stanco. R. Stroili. 11. Turcato. C. Voci
Dipartirnento d i Fisica dell' Uniuersztii criid INFN. Paduvu. Italy 1
APPENDIX C. THE ZEUS COLLABORATION 144
L. ~ d a r n c z ~ k ~ ~ , L. I a n n ~ t t i ' ~ ? B.Y. Oh. J.R. Okrasinski. P.R.B. Saull? W.S. Toot hackerl.'i.
J . J . Whitmore
Pennsyluania State University, Dept. of Physics. Uniuersity Park. PA, USA
Y . Iga
Polytechnic University. Sagarniharu. .lapan
G. D'Agostini. G. Marini. .A. Nigro
Dipartimentu ùi Fisica. Uniu. 'Lu Supienzu' ond IiVFiV. Rome. [tnly 1
C. Cormack. J.C. Hart. S.A. !dcCtibbin. T.P. Shah
Rutherford Appleton Laboratoq. Chilton. Dzdcot. &On. II. K. "
D. Eppmon . C. Heuscti. H.F.-\\.. Sidroziriski. -4. Sridm. R. \\'ichiriann. D.C. C\'illiirrris
Unioe~sztg of Calzjornia. Santa Cm-. CA. W.4
5 . Pave1
Fachbereich Phgsik der Lini~ue~.sitüt-Ge~su7nt~~0c~t.~~:i~~1~1~ Siegen. Gsrmuny '
H. . - \brarnowic~~~ . S. Daganz8. S. Kaiianov2*. .A. Krrisel. A . Levf'"
Rqrnond und Beverly Sackler Facalt;~/ of Emçt Sçicnces. School o j Pl~jszcs. TeL.4 I,W
Uniuersity. Tel-rlviv. Isruel '
T. Abe. T. Fusayasu. K. Cmemori. T. 'i'arnashita
Department o j Phpsics. Untversity of Tokyo. Tukgo. .lupan
R. Hamatsu. T. Hirose. 51. inuzuka. S. Kitarnura? TT. Xishimura
Tokyo Metropolztan Uniuerssit y. Dept. o j Phl/sic.s. Tokyo. .lapan
APPENDIX C. THE ZEUS COLLABORATION
hl. ArneodoJ0? Y. Cartiglia. R. Cirio. SI. Costa. 11.1. Ferrero. S. hlaselli. Y. llonaco.
C. Peroni. hl. Ruspa. R. Sacchi. A. Solario. A. Staiano
Università. di Torino. Dipartinrento di Fiszca Sperirnentale and INFiV. Tonno. Itulg f
D.C. Bailey C.-P. Fagerstroem. R. Giilea. T. Koop. G.U. Levmari. .J.F. Martin. A. Ilirea.
R.S. Orr. S. Polenz. A. Sabetfakhri, D. Sinimons
University of Toronto. Dept. of Physics. Toronto. Ont.. Canada *
. J . X Butterworth. C.D. Catterall. L E . Hqcs . E.A. Heaphy. T.K. .Jones. .].B. Laiie.
B..]. West
u'niversity Colleye Londorr. Ph?~~sics und .~ .~1ror~orr~j Depl . . Londur~. l:.h'. "
.J. Ciborowski. R. Ciesielski. G. Grzclak. R..J. Xowak. .T.U. Pawlak. R. Pawlak. B. Srrial-
ska.
T. Tymieniecka. ;\.K. M-roblewski. .I.A. Zakrzewski. A.F. Zarriccki
CVarsaw Unzversity. Institute of Eqenrnerr td Ph!j.slc.s. CVursaw. Polrind
M. Adamus. T. Gada-j
Institute for iVuclear Studies. LVursu~w. Pulard
O. Deppe. Y. Eisenberg. D. Hochrnan. L. I<arslion2'
Mieizrnann Institute. Depurtnient of Pmtzcle Plr!ysics. Rehovot. Israel '
K.F. Badgett. D. Chapin. R. Cross. C. Foiidas. S. Nattingly. D.D. Reeder. W H . Smith.
A. Vaiciulis3'. T. \VilcIschek. 'il. LVoclarczyk
University of Wisconsin. Dept. of Physics. Maifison. LW. USA
A. Deshpande. S. Dhawan. V.W. Hughes
Yale Lrniuersity, Department of Phgsics. iVe,~u Haven. CT. USA *
APPENDIX C. THE ZEUS COLLABORATION 146
S. Bhadra. C. Catterall. J.E. Cole, W.R. Frisken. R. Hall-Wilton. hl. Eihakzad. S. Menary
York University, Dept. of Physics. Toronto. Ont.. Canada a
l now visiting scientist at DESY
now at L'niv. of Salerno and INFX Sapoli. Italy
hoow at CERX
' now a t CalTech. CSA
" now at Sparkasse Bonn. Gerrnany
"oow a t Siemens ICN. Berlin. Gcrnianny
' ret ireci
'bsiipported by the GIF. contract 1-523- 13-7/97
"PARC Advanced fcllow
'O supported by the Portiiguese Foiintlatiori for Science and Technology
l 1 now a t Dongshin Cniversity. Xajii. i<orea
l 2 now at Massachiiset ts Instit ut^ o f Tditiology. Cattibriclge. II.\. CSA
l h o w at Fermilab. Batavia. EL. CS.4
la' cleceased
'" now at SAP A.G.. &lldorf. Gerriiariy
l6 now a t Netlife AG. Hamburg. Gerniany
l Ï now at Cniversity of Edinburgh. Edinburgh. C.K.
l8 supported by an EC fellowship iitimber ERBFSIBICT 972523
visitor of Cniv. of Crete. Greece. partially siipportrd by DAAD. Bonn - Iiz. .4/98/1676-4
" on leave from MSC. supported by the GIF. contract 1-0444-176.07/95
'' supported bu DAAD. Bonn - Iiz. ;\/98/12712
'* aalso a t University of Tokyo
23 supported by the Cornunidad Autonoma de Madrid
'%ow at Loma Linda Cniversity. Loma Linda. CA. CSA
APPENDIX C. THE ZEUS COLLABORATION 147
" supported by the Feodor Lynen Program of the Alesander von Humboldt foundation
xi partly supported by Tel Aviv University
2i an Alexander von Humboldt Fellow at Cniversity of Hamburg
supported by a MINER\:\ Fellowship
'M present address: Tokyo 'iletropolitan University of Healt h Sciences. Tokyo 1 16-855 1.
.Japan
" now also at Cniversità del Piemonte Orientale. 1-28100 Xovara. Italy
" noiv at University of Rochester. Rochester. SI-. CS;\
supported by the Yatural Sciences and Engineering Research Coiincil of
Canada (XSERC)
supported by the FCAR of Québec. Canada
supported by the German Federal Mnistry for Education and Scimce.
Research and Technologv (BS LBF). uncler cont ract nurnbers OfiTBN NP.
05ïFR19P. OXHH19P. OXHHLSP. O 5 l S I I 3
supported I>v the MINERVA Cesellschaft für Forschung GtnbH. the Gernian
Israeli Foundation. the Israel Science Foiinciation. the C.S.-israel Binational
Science Foundation. the Israel Mnistry of Sc-iencc arid the Benozyio Center
for High Energy Physics
supported by th^ Gernian-Isradi Foiiridat ion. t htl Israel Sciencr Foiindiitiori.
the C .S.-Israel Biriat iorial Scieriw Foii~daciori. ;met by t hc Israr4 Miriist.ry of
Science
supported by the Italian National institiitr for Suclear Physics ( I N F N )
supported by the .lapanese Slinistry uf Education. Science and Cuittire ( the
Monbusho) and i ts grants for Scierit ific Rcsearch
supported by t hc Koreari Slinist ry o f Edi iw iori arid Korea Scieric~ ancl Erigi-
neering Foundat ion
supported by the Setherlands Foundation for Research on SIatter (FOSI)
supported by the Polish State Conimittee for Scientific Research. grant No.
i 12/E-356/SPLB/DESk'/P03/DZ 3/99. G'LOIE-77/SPCB/DESY/P-031 DZ
1/99. 2P03B03216. 2P03BO.1616. 2P03B03517. and by the Gerrnan Federal
Ministry of Education and Science. Rcsearch and Technolog' (BSIBF)
APPENDIX C. THE ZEUS COLLABORAT~ON
supported by the Polish State Comniittee for Scientific Research ( g a n t No.
'2P0380861-1 and 2PO3BO6116)
partially supported by the German Federal Mnistry for Education and Science.
Research and Technology ( BhIBF)
supported by the Fund for Fundamental Research of Russian Slinistry for
Science and Education and by the Gcrniaii Federal SIinistry for Education
and Science. Research and Teclinology ( BSIBF)
supported by the Spanish Mnistry of Editcrrtion ancl Science tlirough fiincls
provideci by CICYT
supported by the Part icle Physic.s ;itid Ast rorioniy Researcli Coiiric-il
supported by the US Departnient of Etiergy
Appendix D
Glossary
BAC : Backing Caloririit?ter.
BCAL : Barre1 Calorimeter.
BMUON : Barrel !duon chambers.
CAL : Calorimeter.
CC : Charged Current.
CTD : Central Tracking Detector.
DAQ : Data Acquisition System.
DESY : Deutsches Elektronen Synchrotron. t h t h Gernian national high energ? physics
laboraton located in Hamburg. Gerrriany.
DIS : Deep Inelastic Scattering.
DST : Data Summary Tape.
EMC : Electromagnetic section of the CAL.
EVB : Event BuiIder.
FCAL : Forward Calorirneter.
FDET : The combination of the FTD and TRD.
FMUON : Forward Muon chanibers.
FTD : Fonvard Tracking Detector.
GFLT : Global First Level Trigger
GSLT : Global Second Level Trigger.
HAC : Hadronic section of the CAL
HERA : Hadron Elektron Ring Ariliigc.
LHC : Large Hadron Collidcr.
LSP : Lightest Supersymnwt ric Piirticitl.
LUMIE : Part of the luminosity irioriit or for riirasilring Elcct rom from t tic Bct tic-Heit lcr
process.
LUMIG : Pan of the luminosity monitor for nieasuring Gammas from the Bethe-Heitler
process.
-UPlonck: The Planck Scale. If distance scales become &on enough (of atornic dimensions
or smaller). the theory of quantum mechanics must be used. Therefore. as one
extrapolates back in time to the beginning of the Cniverse, eventually one would
reach a state of sufficient temperature and density that a fully quantum mechanical
theory of gravitation would be required. This is called the Planck era. and the
corresponding scales of distarice. energ-. and timc are called the Planck scale.
MC : Monte Carlo.
MOZART : SIonte Carlo for ZELS Analusis. Reconstruction and Trigger.
MSSM : The Minimal Supersymrrictric estension to the Standard .\loclel of piirticlr
p hysics.
NC : Seutral Currerit.
NWA : Yarrow Width Approsirriatiun.
PDF : Parton Density Fiinctio~i.
PHYTIA : Monte Carlo simtilation iiiiploriicwtiiig a string mode1 for hatlronizatiori.
PMT : P hotornultiplier Tube.
QCD : Quantum Chromo-Dynaniics.
QFT : Quantum Field Theory.
RCAL : Rear Calorimeter.
RAMUON : Rear Uuon charnbers.
RTD : Rear Tracking Detector.
SM : Standard Model of particle physics.
SPYTHIA : Supersymmetric extension of PYTHIA simulating superqmmetric parti-
cles and t heir interactions.
SUSY : Supersymmetry.
SUSYGEN : blonte Carlo simulation for SLSY interactions at e ie - and ep colliders.
TLT : Third Level Trigger.
TLTZGANA : The offline TLT sirnuiatiori.
TRD : Transit ion Radiation Detector.
TOE : Theory of Everything.
UCAL : Craniurn Calorinleter.
VCTRA K : CTD Track reconst riir t ion packiigt..
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