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Folie%201

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Machine induced background in ALFA. The ALFA detector. elastic scattering ... beam halo from collimation inefficiencies. betatron cleaning. momentum cleaning ... – PowerPoint PPT presentation

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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
2
Forward 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
4
elastic scattering
5
special 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)

6
Simulation 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
7
Simulation 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

8
luminosity 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
9
Performance estimation systematic uncertainties
Recent work obtained for the ALFA TDR (in review)
Background contribution
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

11
beam 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

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

single rates
  • accidental coincidence rate inside detector
    acceptance of about 9 Hz (elastic 27 Hz)
  • potentially dangerous since all at small t

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
14
background 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

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 ?L/L 1.1-1.5
  • Total systematic error 2.2-2.6
  • Total error 2.8-3.2

16
local inelastic beam-gas background
The comparison of the rate of distant and local
beam-gas background shows that the latter
contribution can be neglected.
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 ...

18
from Vincent Hedberg
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)?
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