machine induced background in alfa the alfa detector elastic scattering and luminosity
DESCRIPTION
Machine induced background in ALFA The ALFA detector elastic scattering and luminosity background generation, rejection and subtraction impact on luminosity determination Conclusion & open issues. Hasko Stenzel Background WG meeting. Forward Roman Pots for ATLAS. ATLAS. 240 m. ALFA. - PowerPoint PPT PresentationTRANSCRIPT
Machine induced background in ALFA
• The ALFA detector• elastic scattering and luminosity • background generation, rejection and subtraction• impact on luminosity determination• Conclusion & open issues
Hasko StenzelBackground WG meeting
ALFA background H.Stenzel, 16.03.07 2
Forward Roman Pots for ATLAS
240 mATLAS ALFA
ALFA background H.Stenzel, 16.03.07 3
The ALFA detector
RP
IP240m 240m
RPRP RP
RP RP RP RP
PMT baseplate
optical connectors
scintillating fibre detectors glued on ceramic supports
10 U/V planesoverlap&trigger
Roman Pot
MAPMTsFE electronics
& shield
Roman Pot Unit
ALFA background H.Stenzel, 16.03.07 4
elastic scattering
ALFA background H.Stenzel, 16.03.07 5
Transversal displacement of particles in the ring away from the IP:
Special optics with high * and parallel-to-point focusing:
independent of the vertex position
properties at the roman pot (240m)
y*
y*
parallel-to-point focusingydet
IP Leff
advance phase 90 ,point -to-parallel
; o
2**,
**det ptLy yyeffy
special optics: high ß*
1227
*y
1009
0m 119
m 2625
scmL
***** sinsincos yyy
m rad1GeV 098.0
GeV 0004.0rad 7.44
rad 7.2mm 12.0
2max
2min
max
min
d
N
tt
ALFA background H.Stenzel, 16.03.07 6
Simulation set-up
elastic generatorPYTHIA6.4
with coulomb- and ρ-termSD+DD non-elastic
background, no DPE
beam propertiesat IP1
size of the beam spot σx,y
beam divergence σ’x,y
momentum dispersion
beam transportMadX
tracking IP1RP high β* optics V6.5
including apertures
ALFA simulationtrack reconstruction
t-spectrumluminosity determinationlater: GEANT4 simulation
ALFA background H.Stenzel, 16.03.07 7
Simulation of elastic scattering
2
,
2
,
2
2222*
yeffxeff
yx
Ly
Lxp
ppt
t reconstruction:
hit pattern for 10 M elastic events simulated with PYTHIA + MADX for the beam transport
2
sin
effL
special optics parallel-to-point focusing high β*
ALFA background H.Stenzel, 16.03.07 8
luminosity determination
input fitStat.error
L 8.10 1026 8.151 1026 1.77 %
σtot 101.5 mb 101.14 mb 0.9%
B 18 Gev-2 17.93 Gev-20.3%
ρ 0.15 0.143 4.3%
Simulating 10 M events,running 100 hrsfit range 0.00055-0.055
ALFA background H.Stenzel, 16.03.07 9
Performance estimation: systematic uncertaintiesRecent work obtained for the ALFA TDR (in review)
Backgroundcontribution
ALFA background H.Stenzel, 16.03.07 10
background considerations
● physics background: single diffraction
• can be rejected by means of vertex and acollinearity cuts• is reduced to a negligible level
● machine background
• beam halo originating from cleaning inefficiencies and distant quasi-elastic beam gas interactions, calculations were provided by Igor Bayshev, IHEP
• local inelastic beam-gas interactions (showers), calculations were provided by Igor Azhgirey, IHEP
ALFA background H.Stenzel, 16.03.07 11
beam halo
Calculations are carried out for the high β*-optics with εN =1μrad m and at L=1027cm-2s-1
● beam halo from collimation inefficiencies
• betatron cleaning• momentum cleaning
● halo beam-gas interactions
• elastic and quasi-elastic p-N interactions
ALFA background H.Stenzel, 16.03.07 12
beam halo background
● distributions of halo impacts in the transversal plane at the detector
● normalized per proton hitting a collimator/interacting with beam gas
● This can be turned into single and accidental coincidence rates by
● main question: what is the lifetime contribution for beam gas?
• 100 hrs for MC & BC• 1000 hrs for beam gas
bunchbsacc
partbunchbbs
tRR
kkNR
2
● accidental coincidence rate inside detector acceptance of about 9 Hz (elastic: 27 Hz)
● potentially dangerous since all at small t
2-1-27
10
cms10Ls021.2
1043
bunch
part
bunch
tkk
single rates
ALFA background H.Stenzel, 16.03.07 13
beam halo rejection cuts
Exploit back-to-back signature of elastic events and vertex reconstruction
after vertex and acollinearity cuts still 140 k events survive!(compared to 6.6 M elastic signal)
irreducible background at small t in the luminosity region!
must be subtracted
ALFA background H.Stenzel, 16.03.07 14
background calculation
RP
IP
signal & background in asymmetric configuration
240m RPRP RP
RP RP RP RP
240m
pure background
● signal and irreducible background appear in asymmetric configurations: +/- and -/+
● pure background is also present in symmetric configurations +/+ and -/-
● from this the irreducible background can be calculated by inverting randomly (left/right) the vertical sign of the hits
● halo asymmetries can be corrected for using data
● free of MC, good systematics
ALFA background H.Stenzel, 16.03.07 15
systematic uncertainty of background
● In principle the method is free of syst. uncertainties, since all is determined from the data itself
● However, the calculated background sample is subject to statistical fluctuations, i.e. the subtraction not exact.
● this effect is estimated by generating a large number of background sample with equal statistics and applying the subtraction procedure. In the end the RMS of the fitted luminosity results is quoted as syst. error.
● Result: ΔL/L = 1.1-1.5 %
● Total systematic error: 2.2-2.6 %
● Total error : 2.8-3.2 %
ALFA background H.Stenzel, 16.03.07 16
local inelastic beam-gas background
The comparison of the rate of distant and local beam-gas background shows thatthe latter contribution can be neglected.
ALFA background H.Stenzel, 16.03.07 17
conclusion
● ATLAS proposes to determine the absolute luminosity using elastic scattering in the Coulomb-Nuclear interference region measured with the ALFA subdetector
● The success of this measurement depend crucially on the beam conditions
● The background calculations provided by IHEP Protvino constitute an essential element in the performance estimation
● A precision of about 3% for the luminosity is within reach
● Other methods for the luminosity determination (W/Z counting, optical theorem, ..) are in parallel pursued
● Open issues : beam-gas background for LUCID ...
ALFA background H.Stenzel, 16.03.07 18
from Vincent Hedberg
ALFA background H.Stenzel, 16.03.07 19
open issue: beam-gas background for LUCID
● The beam-gas background entering LUCID from the back has been estimated to be at a small level
● The beam gas entering LUCID from the front is presumably rather small (length ratio) but could be dangerous, since it is pointing to LUCID
● Can we get a background calculation for this contribution at a scoring plane of the LUCID front face (~17m)?