Prsentation PowerPoint

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ATLAS Forward Physics program
Ljiljana Simic Institute of Physics,
Belgrade
Workshop of the Collaboration on Forward
Calorimetry at ILC
22-24 September 2008 Vinca Institute
of Nuclear Sciences, Belgrade, Serbia
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ATLAS Collaboration Expected Performance of
the Detector, Trigger and Physics
CERN-OPEN-2008-020, Geneva, 2008, to appear.
arXiv
2008 JINST 3 S08003
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ATLAS Forward Physics Program

• Absolute and relative Luminosity measurements
• Elastic pp scattering at very small angles
• Diffractive measurements with early data
• Soft single diffraction (SD), Central
  exclusive di-jet production (CEP)
• single diffractive dj-jet production
• Forward physics program will be extended at high
  luminosity
• new physics in CEP
  ,XH, ??, jet-jet, rapidity gap
• exclusive Higgs production, W pair
  prod

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Forward Detectors in ATLAS

• In addition to main ATLAS detector also
  three smaller systems are built to cover the
  forward region.
• LUCID (Lminosity measurement Using Cerenkov
  Integrating Detector)
• is dedicated to online luminosity
• monitoring and is located at 17m from IP1
  (near TAS collimator).
• ZDC (Zero Degree Calorimeter) located at a
  distance 140 m , from IP1, where LHC beam pipe is
  divided into two separate pipes.
• This detector is dedicated mainly to HI.
• ALFA system (Absolute Luminosity For ATLAS)
  located in roman pots at distance 240 m on each
  side of the IP.
• Additionally, proton tagging detectors and
  radiation hard detectors at 420 and 220m from
  IP1 are considered as a part of possible upgrade
  of forward physics program.
• These are dedicated entirely to diffractive
  physics studies.

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Physics Interest in Luminosity

• A precise determination of the luminosity is
  important at the LHC !
• Represents one of the main systematic error in
• cross section measurement, Higgs couplings,
  Top couplings, TGCs)

CSC note on Top cross-section
If all systematic in cross section measurement is
under control any deviation from the predicted
value will be sign of new physics.
Contribution of SUSY background to top pair
production
There are two kinds of luminosity
measurements Abosulte value serves as a
reference point Relative allows to follow values
of accumulated luminosity as a function of time
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LUCID
LUCID is dedicated to online luminosity
monitoring
Consists of 20 aluminium tubes (1.5 m long) which
surrond beam pipe at z /- 17 m from IP
(5.4lt?lt6.1). Tubes are filled with
perfluorobutan (C4F10) gas- pressure 1-2 bar.
When charged particles go trough tube Cerenkov
light is emitted, focused with the Winstone cones
and read out by rad hard PMT. Time resolution of
the detector is of the order of 140 ps which
allows to determine the luminosity bunch by
bunch.
Two LUCID vessels ready to be installed in ATLAS.
Detector is installed in June 2008.g.
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Luminosity monitoring
By observing the change in the mean number of
hits per tube LUCID can determine the change in
luminosity. For an absolute measurement of
luminosity LUCID must be calibrated with known
luminosity. It will proceed in steps. At firs,
using LHC machine parameters (accuracy 10-20.
with special effort) In addition Z or W bozon
events can be used as the production
cross-section is well known (5-8 ) ALFA
calibration during special optics3
There is linear dependence between luminosity and
number of tracks counted in the detector.
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Luminosity measurements
Luminosity determined from the measurement of
the LHC beam parameters
Nxi number of protons in bunch i of beam x
frevolution frequency sx,sytransverse beam
dimensions at the IP Kb number of bunches
bb function at IP eNsxsyg/b normalized
emittance gE/mp (7460)
Accuracy limited by Precision
in measurement of bunch currents Extrapolation of
sxsy from measureament point to IP Beam-beam
effects at IP, beam crossing angle, ...
Accuracy from the beam parameters - Early
running 20-25 - Using special calibration
runs with simplified machine parameters get
to 10 or better - Previous experience at
hadron colliders 510
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Luminosity measurements

• In addition, production of Z or W bozons can be
  used since this processes have well known
  cross-section.
• This method will provide accuracy in absolute
  luminosity measurements
• of 5-8 .
• It is possible to use exclusive muon pair
  production by double photon exchange.
• pp-gtppµµ (cross section µbarns)

Dielectron invariaprnt mass distribution in
Z-gtchannel for 50 pb-1
Overall uncertainty in W cross section
measurement of 5 and in Z cross section of
3 can be achieved with 50 pb -1.
The goal of ATLAS is to reach precision for
absolute L measurement of 3
To achieve this ATLAS aim to extract the absolute
luminosity from measurement of the elastic pp
scattering in the Coulomb-Nuclear interference
(CNI) region.
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Luminosity measurements

• momentum transfer -t (pq)2
• q beam scattering angle
• p beam momentum

ds/dt (mb/GeV2)
When transverse energy at the proton vertex is
close to 0 the t distribution of the elastic
scattering cross section is given by formula
From fit to data in CIN region it is possible to
determine L, stot , b, and r. (UA4)
CNI region fC fN ? _at_ LHC -t 6.5 10-4
GeV2 qmin3.4 mrad
(qmin120 mrad _at_ SPS, UA4 Coll, precision
3) Measurement very challenging!
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ALFA Detector

• To measure absolute luminosity via elastic
  scattering at small angles
• (down to about 3 µrad) ATLAS will use ALFA
  detector.
• System consists of scintillating fiber trackers
  located in Roman pots which allow detectors to
  approach very close (1-2 mm) to beam.
• Specially prepared beam conditions are required
  high-beta (ß) optics and reduced beam emittance.
  
• The main requirements on the tracker are
  resolution below 100µm
• and sensitive edge region

Schematic layout of the ALFA detector in Roman pot
ALFA detector final installation and first
measurements in 2009.
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ZDC zero degree calorimeter
To separate particles produced at 0 degree from
beam ZDC will be installed at place where beam
pipe is separated into two pipes.

• ZDC will detect forward neutral particles with
  ?gt8.3.
• At LHC star-up (pp collisions) ZDC will increase
  the acceptance of ATLAS central and forward
  detectors and provide additional minimum biase
  trigger
• reduce backgrounds produced from beam-gas
  and beam-halo effects
• For HI collisions, ZDC will provide low rate
  trigger for ultra peripheral collisions and will
  measure collision centrality.

6 tungsten-quartz calorimeter modules 1 EM and 3
Hadronic
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Summary

• Luminosity
• LUCID and ALFA will provide measurement of
  luminosity at ATLAS with accuracy better than 5.
• Forward Particle Spectrum
• ZDC will measure forward particle production
  for MC tuning.
• ZDC will measure forward spectators for HI
  collisions, it will provide low rate trigger
  for ultra peripheral collisions and will
  measure collision centrality.
• Low Luminosity Physics
• ALFA will measure elastic scattering and
  s(tot).
• single
  diffractive forward proton spectrum
• with rapidity gap veto in FCAL, LUCID, ZDC
  single diffractive di-jet and W production,
  central exclusive production of di-jets will be
  measured.
• High Luminosity Physics
• Possible upgrade with installing radiation
  hard tracking detectors at 220 and 420 from IP1
  will provide good measurement of new physics.

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