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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 Presentation
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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)?