Title: Folie%201
1- 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
2Forward Roman Pots for ATLAS
ATLAS
240 m
ALFA
3 The ALFA detector
RP
RP
RP
RP
240m
240m
IP
RP
RP
RP
RP
MAPMTs FE electronics shield
PMT baseplate
optical connectors
scintillating fibre detectors glued on ceramic
supports 10 U/V planes overlaptrigger
Roman Pot Unit
Roman Pot
4elastic scattering
5special optics high ß
- 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) -
6Simulation set-up
elastic generator PYTHIA6.4 with coulomb- and
?-term SDDD non-elastic background, no DPE
beam properties at IP1 size of the beam spot
sx,y beam divergence sx,y momentum dispersion
ALFA simulation track reconstruction
t-spectrum luminosity determination later
GEANT4 simulation
beam transport MadX tracking IP1?RP high ß
optics V6.5 including apertures
7Simulation of elastic scattering
hit pattern for 10 M elastic events simulated
with PYTHIA MADX for the beam transport
t reconstruction
- special optics
- parallel-to-point focusing
- high ß
8luminosity determination
Simulating 10 M events, running 100 hrs fit range
0.00055-0.055
input fit Stat. error
L 8.10 1026 8.151 1026 1.77
stot 101.5 mb 101.14 mb 0.9
B 18 Gev-2 17.93 Gev-2 0.3
? 0.15 0.143 4.3
9Performance estimation systematic uncertainties
Recent work obtained for the ALFA TDR (in review)
Background contribution
10background 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
11beam halo
- Calculations are carried out for the high
ß-optics with - eN 1µrad m and at L1027cm-2s-1
- beam halo from collimation inefficiencies
- betatron cleaning
- momentum cleaning
- halo beam-gas interactions
- elastic and quasi-elastic p-N interactions
12beam 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
single rates
- accidental coincidence rate inside detector
acceptance of about 9 Hz (elastic 27 Hz) - potentially dangerous since all at small t
13beam 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
14background calculation
RP
RP
RP
RP
240m
240m
IP
RP
RP
RP
RP
signal background in asymmetric configuration
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
15systematic 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 ?L/L 1.1-1.5
- Total systematic error 2.2-2.6
- Total error 2.8-3.2
-
16local inelastic beam-gas background
The comparison of the rate of distant and local
beam-gas background shows that the latter
contribution can be neglected.
17conclusion
- 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 ...
18from Vincent Hedberg
19open 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)?