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Proton structure functions at HERA
Tuning, N.
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Citation for published version (APA):Tuning, N. (2001). Proton structure functions at HERA.
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Download date: 07 Sep 2020
Chapterr 3
HERAA and ZEUS
3.11 Introductio n
Thee Hadron Elektron Ring Anlage (HERA) is a unique collider, accelerating particles withh different masses. Protons are accelerated to an energy of 820 GeV, and positrons are acceleratedd to 27.5 GeV. Interactions take place at four places along the ring, see Fig. 3.1.
Att two intersections, in the north and south area, the beams are collided head on pro-ducingg a centre-of-mass energy of <<Js — y/4EeEp = 300 GeV. The fragments of the ep collisionss are detected by the HI and ZEUS experiments. The main aim of the two colliderr experiments is the study of the structure of the proton.
InIn the east hall, the protons are collided on a fixed target, consisting of 2 x 4 wires in the haloo of the proton beam. The HERA- B experiment measures J/ip's originating from 6-decayss with the aim of measuring CP violation in the B-system. In the west hall the positronss are longitudinally and transversely polarised and subsequently collided with polarisedd gas. The HERME S experiment measures the spin-dependent structure of the proton. .
Inn this chapter the HERA accelerator and the components of the ZEUS detector, used in thiss analysis, are discussed. In the last section of this chapter the event simulation, both thee event generation as well as the detector simulation, is presented.
3.22 The HERA accelerator
Thee protons are acquired by ionising hydrogen, obtaining H~. The hydrogen ion source usedd is the magnetron, originally known as planotron [26]. The ionised hydrogen is subse-quentlyy accelerated and the two bound electrons are stripped off by passing through a gold foil .. The positrons are obtained by conversion of photons produced by Bremsstrahlung
21 1
22 2 ChapterChapter 3 HERA and ZEUS
Figuree 3.1: The HERA accelerator and its experiments.
inn an electron beam. The electrons and positrons are separated with a magnetic field.
Thee charged particles are preaccelerated by a system of 3 linear accelerators (LINAC) andd injected into the Positron Elektron Tandem Ring Anlage (PETRA), see Fig. 3.1 The protonss are accelerated to 40 GeV in PETRA, whereas the positrons reach 12 GeV before injectionn into HERA (see also Table 3.2). In HERA the positrons are kept in circular orbitt by normal conducting magnets, and the 2000 times more massive protons require superconductingg magnets.
B e amm parameters Circumferencee [m] Bunchh spacing [ns] Bunchh spacing [m] Numberr of buckets Beamm crossing angle [mrad]
Centree of mass energy [GeV] Numberr of colliding bunches Peakk luminosity [cm~2s_1] Specificc luminosity [A _ 2cm_ 2s- 1] Integratedd luminosity [pb^y r '1 ] Widthh of interacting beams [mm] Heightt of interacting beams [mm]
19966 1997 Design 6335.83 3
96 6 28.8 8 220 0 0 0
300.9 9 174 4 1.03-1030 0
5.4-1029 9
17.2 2 0.200 0 0.054 4
300.9 9 185 5 1.40-1031 1
6.0-1029 9
33.2 2 0.200 0 0.054 4
313.7 7 210 0 1.58-1031 1
3.58-1029 9
53.169 9 0.247 7 0.078 8
Tablee 3.1: HERA parameters for colliding positron-proton beams in 1996 and 1997 and
thethe design values.
3.23.2 The HERA accelerator 23 3
HERAA luminosity 1993-97
. Q Q Q . .
CO O O O C C
I I _ l l
CD D
08 8
O) ) CD D
1500 200
Dayss of running
Figuree 3.2: The machine performance improved during the years 1993 to 1997, indicated byby the increase in delivered luminosity.
Thee key parameter of particle colliders is the luminosity C. The higher the luminosity, the largerr is the event rate for a certain process. High luminosity is obtained by optimising thee transverse size of the beams, the number of particles in a bunch, the lifetime of the beamss and the number of bunches circulating in the machine:
££ = ƒ 47TtT,<7„„ '
(3.1) )
wheree n\ particles collide on n2 particles with frequency ƒ and where ax and av are the Gaussiann transverse beam profiles in the x and y direction [10].
Usuallyy the beam size is expressed in terms of the transverse emittance e, denoting the beamm quality, and the amplitude function /3, determined by the magnet configuration. Thee emittance, /3-function and the beam width are related through:
7T(T T
T T (3.2) )
Thee parameters for e+p collisions are listed in Table 3.1, whereas positron and proton specificc parameters are listed in Table 3.2. The values achieved in 1996 and 1997, as well ass the design values are listed.
24 4 ChapterChapter 3 HERA and ZEUS
Beamm parameters
Nominall energy [GeV] Relativee Energy spread (AE/E) Injectionn energy [GeV] j3j3 = v/c 77 = E/m Max.. circulating current [mA] Averagee initial current [mA] Particless per bunch Synchrotronn radiation loss
perr turn per particle [MeV] Synchrotronn radiation power [MW] RFF frequency [MHz]
Positron n 1996/1997 7 27.567 7
12 2 1 -3 -10-10 0
55000 0 42.7 7 33.0/36.0 0 2.9-1010 0
499.667 7
Design n 30 0 1.02-10-3 3
12 2
58 8 58 8 4.041010 0
117 7 6.81 1 499.667 7
Proton n 1996/1997 7 820.977 7
39.79 9 11 - 10~6
875 5 105 5 65.0/76.8 8 7.7-1010 0
522 - 208
Design n 820 0 9.47-10"5 5
40 0
160 0 160 0 11.2-1010 0
5.93-10-6 6
9.48-10"7 7
522 - 208
Tablee 3.2: HERA parameters for the positron and proton beams separately, in the years 19961996 and 1997 and the design values.
Sincee the start of HERA in 1992, the machine increased its performance (see Fig. 3.2), reachingg a luminosity of 38 p b_ 1 in 1997, close to the design value. The luminosity avail-ablee to the experiments however, is less than the delivered luminosity due to background conditionss at the beginning of a fill. In the large shutdown in early 2001 the machine wil ll be upgraded. The increase in luminosity is foreseen to amount to a factor 3-5. It is foreseenn that by 2005 the integrated luminosity wil l reach 1 fb_1.
Inn 1996(1997) HERA operated with 174(185) colliding bunches of 820 GeV protons and 27.55 GeV positrons, with a time between bunches of 96 ns. In addition unpaired positron, unpairedd proton and empty bunches are available to study beam related backgrounds, seee Section 5.2.2. The proton bunch length was approximately 11 cm (r.m.s.) while thee positron bunch length was negligible in comparison which, together with run-to-run variationss of the mean interaction position, leads to a length of the interaction region of 11.55 cm (r.m.s.) centred around Z—01.
Approximatelyy 6% of the proton current was contained in satellite bunches, which were shiftedd by 8 ns with respect to the primary bunch crossing time. This resulted in a fractionn of the ep interactions occurring at {Z) — 2 cm (corresponding to the neigh-bouringg RF bucket, ƒ = c/(2 * 0.72 m) - 208 MHz).
^ hee HERA coordinate system is a right-handed Cartesian system, with the Z axis pointing in the protonn beam direction, referred to as the "forward direction", and the X axis pointing left towards the centree of HERA. The coordinate origin is at the nominal interaction point.
3.33.3 The ZEUS detector 25 5
3.33 The ZEUS detector
Thee ZEUS detector is a traditional all-purpose detector, with tracking detectors close to thee interaction point to detect the path of charged particles, with calorimetry surrounding itt to measure the energy of particles and with a muon system at the outside of the detector. AA detailed description of the ZEUS detector can be found elsewhere [27]. A brief outline of thee components which are most relevant for this analysis is given below. The components off the ZEUS detector used in this analysis are shown in Fig. 3.3 and schematically in Fig.. 3.8.
RMUON N
Figuree 3.3: Detailed view of the ZEUS detector.
2G G ChapterChapter 3 HERA and ZEUS
3.3.11 Calorimeter
Thee ZEUS calorimeter is a uranium-scintillator sampling calorimeter (CAL) [28]. The high-resolutionn calorimeter consists of three parts: the forward (FCAL), the barrel (BCAL) andd the rear (RCAL) calorimeters.
Eachh part is subdivided transversely into towers and longitudinally into one electromag-neticc section (EMC) and either one (in RCAL) or two (in BCAL and FCAL) hadronic sectionss (HAC), see Fig. 3.5. The smallest subdivision of the calorimeter is called a cell. AA tower contains four electro-magnetic cells of typically 5 x 20 cm2 in the FCAL and BCAL ,, and two electro-magnetic cells of 10 x 20 cm2 in the RCAL. The tower contains onee hadronic cell of 20 x 20 cm2 in the RCAL and two in the BCAL and FCAL. Each celll is viewed by two photomultiplier tubes (PMT), thus avoiding "holes" in the detector inn case one of the two PMTs fails. The imbalance between the signals in the two PMTs, II = (-EWt — £"right)/(£ïeft + bright), provides position information on the energy deposit.
Thee single-particle_energy resolutions, as measured under test beam conditions, are a(E)/Ea(E)/E = 0.18/%/Ê1 for electrons and a(E)/E = 0.35/VË for hadrons (E in GeV). AA particle entering the calorimeter starts to shower in the absorber material, the depleted uraniumm (DU), and subsequently ionises the active material, the scintillator. Part of the incidentt energy is transformed into optical photons in the scintillator. The measured en-ergyy is thus proportional to the number of photons in the scintillator. The accuracy of thee energy measurement depends on the number of photons, leading to the square-root behaviour. .
hadronn electron muon
Figuree 3.4: Electro-magnetic particles, hadronic particles and muons shower differently in thethe calorimeter.
3.33.3 The ZEUS detector 27 7
Thee electro-magnetic cascade, initiated by electrons and photons, consists of lower en-ergeticc e+e~ pairs and bremsstrahlung photons and peaks at small depths. Hadronic particless have an electro-magnetic component due to n° production, but also a hadronic component,, depositing energy at larger depths, see Fig. 3.4. Both the hadronic component off the shower and the electro-magnetic component produce a measured signal, which is proportionall to the energy in the shower. In general the constant of proportionality is dif-ferentt for electro-magnetic (e) and hadronic (h) showers. The ratio e/h is generally larger thann 1 [10], compromising the energy measurement, since it becomes dependent on 7r° componentt of the cascade. The ZEUS calorimeter, with 2.5 mm scintillator, sandwiched betweenn 3 mm uranium plates, achieves compensation, e/h = 1.0.
Thee natural radioactivity from the depleted uranium irradiates uniformly all scintillator platess of the calorimeter, thus providing a monitoring and calibration of the scintillator lightt transmission and the gain of the PMTs. The measured uranium activity provides a longg term calibration at the level of 1% [27].
Thee timing resolution of a calorimeter cell is better than 1 ns for energy deposits greater thann 4.5 GeV .
Thee CAL covers 99.7% of the total solid angle. The FCAL covers the polar angle range 2.6°° < 9 < 36.7°, the BCAL covers 36.7° < 0 < 129.1° and the RCAL 129.1° < 6 < 178.5°,, see Fig. 3.5 and Table 3.3. The FCAL and RCAL are divided horizontally into twoo halves to allow retraction during beam injection.
36.7 7 129.1 1
positrons s
= = = = = = = = = = = =
W W bsWi\W\\\Vi\\\ill l l 11/ / mmmm .
FCAL-EMCC RCAL-EMC
A A
/ /
BCAL-bM C C
^mr/M/miiiihunm ^mr/M/miiiihunm III I \\\ \\\ \w w sw w WW W
y y ' '
— —
7 7 / /
protons s
^ ^ ^ ^ ^
FCAL-HAC C BCAL-HAC C RCAL-HAC C
Figuree 3.5: A schematic picture of the calorimeter. The different EMC and HAC sections areare shown, as well as the angular coverage of the BCAL.
28 8 Chapterr 3 HERA and ZEUS
3.3.22 Tracking Detectors
Centrall Trackin g Detec tor
Chargedd particles are tracked in the central tracking detector (CTD) [29], which operates inn a magnetic field of 1.43 T provided by a thin superconducting coil. The CTD consists off 72 cylindrical drift chamber layers, organised in 9 superlayers, covering the polar an-glee region 15° < 6 < 164°. The transverse-momentum resolution for full-length tracks is <T(PT)/PT<T(PT)/PT = 0.0058pr © 0.0065 © 0.0014/pr, with pT in GeV.
Thee interaction vertex is measured with a typical resolution along (transverse to) the beamm direction of 0.4 (0.1) cm.
Thee CTD is used to measure the track momentum and angle of tracks and to extrapolate themm onto the face of the calorimeter. Particles traversing at least 3 superlayers have a well measuredd track and can be matched effeciently to a calorimeter cluster, see Section 5.4.
Thee CTD determines the reference frame of ZEUS, since this detector has the most accuratee position information. Other detector components, like the calorimeter, and the beamline,, are aligned with respect to the CTD.
Rearr Trackin g Detec tor
Thee rear tracking detector (RTD) [27] is a planar drift chamber, covering the polar angle 160°° < 6 < 170°. The angular coverage of the RTD largely overlaps with both the CTD
Figuree 3.6: The left plot shows the track hits in the 9 superlayers of the CTD and the resultingresulting fitted track, obtained from an event display. The right plot shows the individual wireswires of the CTD. The stereo angles of the superlayers are also indicated.
3.33.3 The ZEUS detector 29 9
Detector r
SRTD D RTD D CTDD sll CTDD sl3 CTDD sl9 RCAL RCAL BCAL BCAL FCAL L
"mi n n
160.5 5 160.0 0 11.3 3 18.4 4 36.1 1 129.1 1 36.7 7 2.6 6
"max "max
178.5 5 170.0 0 168.2 2 160.7 7 142.6 6 178.5 5 129.1 1 36.7 7
Projection n onn RCAL (cm) 4.00 < R < 52.5 26.22 < R < 54.0
RR > 31.0 RR > 52.0 RR > 113.5
44 < R < 183 R>R> 183
Projection n onn FCAL (cm)
--
RR > 44.0 RR > 73.2 RR > 160.4
R>R> 164 100 < R < 164
ZZ position (cm)
ZZ = -148 -1222 < Z < -137 1055 < Z < -100 1055 < Z < -100 1055 < Z < -100
ZZ < -148.5 2055 < Z < -125
Z > 2 20 0
Tablee 3.3: The angular coverage of the different detectors are listed. All values are cal-culated,culated, using the nominal vertex, Z = 0 cm. The corresponding area coverage on the RCALRCAL and FCAL face are given in the fourth and fifth columns. The Z position is listed inin the last column.
andd the SRTD, as is indicated in Table 3.3. The RTD is used to align the quadrants of thee small rear tracking detector relative to each other.
3.3.33 Small Rear Trackin g Detector
Thee small angle rear tracking detector (SRTD) [30] is attached to the front face of the RCALRCAL and consists of two planes of scintillator strips, 1 cm wide and 0.5 cm thick, arrangedd in orthogonal directions and read out via optical fibers and photo-multiplier tubes.. It approximately covers the region of 68 x 68 cm2 in X and Y and is positioned at ZZ = —148 cm. A hole of 8 x 20 cm2 at the centre of the RCAL and SRTD accommodates thee beampipe, see Fig. 3.7. The asymmetric shape is a consequence of the decision in 19955 to move the two halves of the central RCAL module closer to the beam. The beam holee decreased from 20 x 20cm2 to 8 x 20cm2, increasing the acceptance for positrons with smalll scattering angles.
Thee SRTD signals resolve single minimum-ionising particles and provide a transverse positionn resolution of 3 mm. The time resolution is better than 2 ns for a minimum-ionisingg particle. The energy loss in the scintillator of a moderate relativistic particle is fairlyy independent of its energy and is referred to as a mip [10]. Pulse heights in the SRTDD and presampler are given relative to the pulse height of one mip.
Thee SRTD was used for a variety of purposes. They are listed below:
Position reconstruction. The position of the scattered positron is accurately reconstructedd using the SRTD (see Section 4.4.3).
Energy corrections. The energy of the scattered positron is corrected for energy lossess in dead material using the SRTD signal (see Section 4.4.2).
30 0 ChapterChapter 3 HERA and ZEUS
Figuree 3.7: The areas covered by SRTD and rear presampler are shown. The right plot showsshows the front of the RCAL, where the shaded area indicates the area covered by 312 tiles ofof the rear presampler. The arc indicates the inner radius of the BCAL.
•• Trigger veto. Due to the high timing resolution, proton-gas interactions, occurring upstreamm of the detector, are effectively rejected by the SRTD (see Section 5.3).
•• Trigger . One of the triggers, used in the analysis, requires a SRTD signal for events withh positrons at low scattering angles (see Section 5.3).
3.3.44 Presampler
AA presampler (PRES) [31] is mounted in front of FCAL and RCAL. It consists of 20 x 20 cm22 scintillator tiles that match the calorimeter towers. It measures particles originating fromm showers in the material between the interaction point and the calorimeter.
Thee rear presampler is used event-by-event to correct the energy measurement of the scatteredd positron, see Section 4.4.2, resulting in an improved resolution of the energy measurement. .
3.3.55 Hadron Electron Separator
Thee hadron electron separator (HES) consists of a layer of silicon pad detectors. The rear HESS is located in the RCAL at a depth of 3.3 radiation lengths. Each silicon pad has ann area of 28.9 x 30.5 mm2 , providing a spatial resolution of about 9 mm for a single hit pad.. If more than one adjacent pad is hit by a shower, a cluster consisting of at most
3.33.3 The ZEUS detector 31 1
1 1
i i
FCALL '
BCAL L
^ ^ ^ PRES ^
n n
i i
i i
i i RCAL L
(Z=-107m) ) LUM I I
positronn tagger (Z=-355 m)
Figuree 3.8: Schematic view of the ZEUS detector. The detector components used in this analysisanalysis are indicated.
3 x 33 pads around the most energetic pad is considered, which allows reconstruction of thee incident particle position with the improved resolution of 5 mm.
3.3.66 35m Tagger
AA positron tagger is located at Z = —35 m. This electro-magnetic calorimeter consists off a lead-scintillator sandwich surrounded by lead shielding. This detector is used to tagg scattered positrons from events with very low four momentum transfer, Q2 < 0.02 GeV^2.. It aids in identifying fake positrons in the main calorimeter originating from photoproductionn events, see Section 5.2.1.
3.3.77 Luminosity Monitor
Thee luminosity is measured via the bremsstrahlung process, ep —¥ cyp, using a lead-scintillatorr calorimeter (LUMI) [32], The theoretical cross section of the Bethe-Heitler processs [33] is well known [34].
Thee luminosity monitor is positioned at Z = —107 m, see Fig. 3.9, and accepts photons att angles < 0.5 mrad with respect to the positron beam direction. The LUMI photon calorimeterr is also used to tag photons from initial state radiation in DIS events. It hass an intrinsic energy resolution of a{E)jE = 18%/v® G e V - I n i t s operating position, however,, it must be shielded from synchrotron radiation (see Table 3.2) by a carbon-lead
32 2 ChapterChapter 3 HERA and ZEUS
1077 m
positronn beam IP
y-detector r
35mm tagger \ positron beam
Figuree 3.9: A schematic view of the luminosity system, consisting of the Luminosity monitormonitor at Z = —107 m and the positron tagger at Z = —35 m, with respect to the interactioninteraction point (IP).
filter.. This degrades the energy resolution to a(E)/E = 26.5%/y/Ë GeV, as determined fromm the bremsstrahlung data. The position resolution is 0.2 cm in X and Y.
3.44 Data sample
Thee measurement presented in this thesis, is based on two samples; a 'low-Q2 region' (Q(Q22 > 2 GeV2) and a 'high-Q2 region' (Q2 > 30 GeV2). The low-Q2 sample, where the positronss are scattered through smaller angles, was obtained in short dedicated running periodss in 1996 and 1997. Due to the large cross section at low Q2, the collected, limited, da taa sample contains sufficient number of events. The high-Q2 sample corresponds to the totall available luminosity in 1996 and 1997. The luminosity of the two samples for the differentt years is listed below:
LowQ2 2
Highh Q2
Q2-range e QQ22 > 2 GeV2
QQ22 > 30 GeV2
£l996 6 1.488 pb"1 % 6.700 pb " : 1.1%
£l997 7 0.744 pb"1 1.8% 23.922 pb"1 1.8%
•£<tot t
22 pb^1
55 pb"1
3.53.5 Event Simulation 33 3
3.55 Event Simulation
Withh an ideal detector events can be measured, counted and subsequently divided by the luminosityy to obtain the desired cross section. However, in practice the measurement is affectedd by detector effects. Scattered positrons can escape through the beam holes, or can bee absorbed in inactive material located between the interaction point and the calorimeter. Accuratee detector simulation is needed to estimate the acceptance of the detector. The inactivee material and deficiencies in the detector components can furthermore affect the measuredd quantities. These detector effects are determined by the detector simulation.
InIn order to be able to decide whether an observed discrepancy between data and event simulationn is caused by faulty detector simulation or underlying physics processes, all the knownn underlying physics processes need to be simulated properly.
Thee event simulation therefore consists of two steps, the generation and the simulation off all known underlying physics processes, as well as the detector simulation. Depending onn the cross section, events have a certain probability to be produced in a particular part off phase space and, on the detector side, particles have a certain probability to shower in inactivee material. Therefore Monte Carlo techniques are used and the full event simulation iss often referred to as Mont e Carlo simulation (MC).
3.5.11 Event generation
Neutrall current DIS events were simulated using the DJANGO 6.24 program [35]. It consistss of the HERACLES 4.5.2 [36] program, interfaced to the LEPTO 6.5 [37] Monte Carlo. .
Thee CTEQ4D [38] next-to-leadnig order parton density parameterisations were used in thee simulations. HERACLES generated events that are distributed in x and Q2 according to: :
^ 3 FF = f j £ [Y+FTEQiD{x, (f) - r_xF3CTE04D(x, 02)] (1 + M « , Q2)).
i.e.. with the FL = 0. The simulated events are reweighted in the analysis to account for thee contribution from longitudinal photon exchange. The DJANGO event generator uses thee HERACLES program for electroweak corrections of 0(a) , i.e. corrections for initial-andd final-state radiation, vertex and propagator corrections.
Thee DJANGO event generator uses partly modified routines from LEPTO to simulate QCDD effects. This allows for generation of events according to QCD cascade models, likee the matrix element parton shower (MEPS) or the colour dipole model (CDM) [39], ass implemented by the ARIADNE 4.08 [40] program. In the present analysis the colour dipolee model is used for the QCD cascade, since the ARIADNE model provides the best descriptionn of the characteristics of the DIS non-diffractive hadronic final state [41] and
34 4 ChapterChapter 3 HERA and ZEUS
^50000^50000 -
40000 40000
--1 1
r r
jf jf
Data • •• Diffractive MC \\ I Diffr + Non Diffr.
,, i 5050 100 150
FCALFCAL Energy (GeV)
Figuree 3.10: (a) Event distribution of the pseudo-rapidity of the most forward energy depositdeposit with more than 400 MeV. (b) Event distribution of the total energy deposited inin the FCAL. These two event distributions show a clear separation of diffractive and non-diffractivenon-diffractive DIS events, indicating the difference of the hadronic final state of the two processes. processes.
includess all leading order QCD diagrams as implemented in ARIADNE for the QCD cascadee and JETSET 7.4 [42] for the hadronisation.
Diffractivee events, which are characterised by a large rapidity gap in the detector [22, 43], aree simulated within ARIADNE, interfaced to the RAPGAP 2.06/52 [44] program. The latterr assumes that the struck quark belongs to a colourless state (pomeron, P) having onlyy a small fraction of the proton momentum.
Thee diffractive events are distributed in Xf and t according to the following pomeron fluxflux [45]:
f(xf(xvv,t) ,t) xxr r 1.3e e -61 1 (3.3) )
seee Section 2.5.1. Finally, the diffractive and non-diffractive samples were mixed as a functionn of x and Q2 in such a way that the Monte Carlo reproduced the nmax distribution inn the data, see Fig. 3.10. In this procedure the sum of the number of diffractive and non-diffractivee events was kept fixed. Here r\max is the pseudo-rapidity of the most forward calorimeterr cluster with more than 400 MeV. Pseudo-rapidity is defined as:
r)r) = — In I tan - j (3.4) )
Thee FCAL energy distribution was used to check the mixing obtained from the r?„ distribution.. Consistent results were found.
3.53.5 Event Simulation 35 5
Q'LnQ'Ln (GeV')
Q'Q'22 > 0.5 g 2 > i i g 2 > 4 4 QQ22>> 10 Q 2 > 4 0 0 Q22 > 100 QQ22 > 400 highh Q2
<r(nb) )
2188.6 6 1108.8 8 307.40 0 123.44 4 24.600 0 7.5846 6 1.0824 4
## events (xlOOO) DJANGO O
2500 0 1600 0 710 0 310 0 200 0 200 0 200 0 100 0
RAPGAP P 320 0 100 0 100 0 100 0 60 0 60 0 0 0 0 0
r( Pb--DJANGO(int.) )
1.19 9 1.46 6 2.32 2 2.54 4 7.89 9 26.0 0 170 0
»500 0
(1.19) ) (2.65) ) (4.97) ) (7.51) ) (15.4) ) (41.4) ) (212) )
') ) RAPGAP P
0.15 5 0.09 9 0.31 1 0.81 1 2.43 3 7.86 6
0 0 0 0
Tablee 3.4: Generated Monte Carlo samples used in this analysis are listed, with the cor-respondingresponding number of generated events and the the equivalent "luminosity".
AA cut on the yp invariant mass, W > 5 GeV, was applied at the generator level to avoidd the resonance region. Special attention was payed to the region y{\ — x)2 < 0.004, wheree virtual QED corrections can become large and lead to negative non-radiative cross sections,, see Section 5.4.
Twelvee different MC samples with varying Q2 ranges were generated with increasing correspondingg luminosities, running from 1.2 pb"1 for Q2 > 0.5 GeV2 upto 1500 fb_1 for QQ22 > 40000 GeV2, see Table 3.4.
Thee main source of background in the data comes from the few photoproduction interactionss which lead to the detection of a fake scattered positron. Direct and resolved photo-productionn events were simulated using PYTHIA 5.724 [42] with cross sections given by thee ALLM parameterisation [46]. Resolved and direct photoproduction events [47], correspondingg to an integrated luminosity of 0.7 and 2.5 pb - 1 , were generated with y > 0.36. Eventss with smaller y values do not contribute to the photoproduction background (see Chapterr 4).
3.5.22 Detector simulation
Thee output of the event generator is passed to the MOZART program, which has the ZEUSS detector implemented using the GEANT 3.13 program [48]. The trigger is simulated withh the ZGANA package. The output of MOZART has the same format as the data output,, enabling identical event reconstruction for data and simulated events.
Thee organisation of processing of the detector simulation is done by the FUNNEL [49, 24] facility,, that distributes the generated events to 272 different CPUs, located at 14 different institutes,, utilising 6 different platforms. After processing the fully simulated events are collectedd by FUNNEL. The FUNNEL facility routinely simulates 2 million events per week. .
36 6 ChapterChapter 3 HERA and ZEUS
AA particular aspect in the simulation was important in the present analysis of the 1996 andd 1997 data. The simulation of cooling pipes on both the rear and forward side of thee beampipe were not simulated properly. On the rear side the copper paste on the coolingg pipes affect the energy measurement of the scattered positron, or can even absorb thee positron completely. The cooling pipes are located in a different position in Monte Carloo than in data, see Fig. 5.5 in Section 5.4. Initially the cooling pipes on the forward sidee were not simulated at all. As a consequence the hadronic final state in data was lesss collimated around the FCAL beamhole, than in Monte Carlo, resulting in different characteristicss of the hadronic final state in data and Monte Carlo, see also Section 6.4.2.
Afterr proper implementation of the dead material in the simulation, the Monte Carlo describedd the data. A Monte Carlo event, where hadrons interact with the inactive materiall of the cooling pipes, is shown in Fig. 3.11.
Thee vertex distribution used in the simulation was taken from DIS events in which both scatteredd positrons and hadrons were well reconstructed by the CTD. A reweighting proceduree allowed the events to be used for the different vertex configurations encountered
Figuree 3.11: A picture of a Monte Carlo event in the forward part of the detector is shown, obtainedobtained from GEANT. The trajectories of the generated particles are shown by the full andand dotted lines. In this particular event, part of the forward jet showers in the forward beampipe,beampipe, affecting the measurement of the kinematic variables.
3.53.5 Event Simulation 37 7
inn the analysis, see Section 4.6.2.
Thee effects of the uranium radioactivity were studied in randomly triggered events. The energyy deposits in these events originate entirely from the calorimeter noise. The rate andd energy distribution are simulated in the Monte Carlo. The signals are added to those off the simulated physics events.
388 Chapter 3 HERA and ZEUS
