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ATLAS Forward Physics Program - Présentation powerpoint


									ATLAS Forward Physics program

                 Ljiljana Simić
          Institute of Physics, Belgrade

Workshop of the Collaboration on Forward Calorimetry at ILC

                   22-24 September 2008
      Vinca Institute of Nuclear Sciences, Belgrade, Serbia

                      ATLAS Collaboration

                      Expected Performance of
                      the Detector, Trigger and

                      Geneva, 2008, to appear.

                      arXiv …

2008 JINST 3 S08003
                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
     new physics in CEP: pp  p  X  p ,X=H, γγ, jet-jet,      rapidity gap
     exclusive Higgs production, W pair prod…                                    jets

                           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.

                   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
              N  L  A  B                                             Top cross-section

      d dN  dB dL d dA
                 
         N B    L    A
If all systematic in cross section measurement
is under control any deviation from the                 Contribution of SUSY background
predicted value will be sign of new physics.            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
  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 (5.4<|η|<6.1).
Tubes are filled with perfluorobutan
(C4F10) gas- pressure 1-2 bar. When
charged particles go trough tube            Two LUCID vessels ready
Cerenkov light is emitted, focused with     to be installed in ATLAS.
the Winstone cones and read out by rad
hard PMT.
Time resolution of the detector is of the    Detector is installed
order of 140 ps which allows to determine    in June 2008.g.
the luminosity bunch by bunch.
                                  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
                                             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
There is linear dependence between
luminosity and number of tracks counted in   used as the production cross-section is
the detector.                                well known (5-8 % )
                                             ALFA calibration during special optics~3%
                  Luminosity measurements
Luminosity determined from           f ikb1 N 1i N 2i   f ikb1 N 1i N 2i   f kb N2
the measurement of the LHC
                                L                                          
                                   Impact surface           4 x  y
                                                                   * *               *
                                                                               4 N 
beam parameters

          Nxi = number of protons in bunch i of beam x; f=revolution frequency;
          sx,sy=transverse beam dimensions at the IP; Kb = number of bunches; b*=b
          function at IP; eN=s*xs*yg/b* normalized emittance; g=E/mp (~7460)

                      Accuracy limited by
     Precision in measurement of bunch currents
     Extrapolation of σxσy 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 5—10%                                 8
                     Luminosity measurements

    •   In addition, production of Z or W
        bozons can be used since this
        processes have well known cross-

    •   This method will provide accuracy in
        absolute luminosity measurements
        of 5-8 %.
                                                 Dielectron invariaprnt mass distribution
                                                 in Z->channel for 50 pb^-1
    •   It is possible to use exclusive muon
        pair production by double photon          Overall uncertainty in W cross section
        exchange.                                 measurement of 5% and in Z cross section
        pp->ppμμ      (cross section ~μbarns)     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.                                                                              9
                                      Luminosity measurements
                                                         momentum transfer -t ~ (pq)2
d/dt (mb/GeV2)

                                                               q = beam scattering angle
                                                                  p = beam momentum

                                                          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
             dN                                         2 EM  tot          
                                                                               b|t |            region it is possible to
                             L f C  f N        L             (i   )e 2
                                                                                                determine L,  tot , b,
             dt    t CNI                                 t    4                                    and . (UA4)

                  CNI region: |fC| ~ |fN|  @ LHC: -t ~ 6.5 10-4 GeV2; q min~3.4 mrad
                                             (q min~120 mrad @ SPS, UA4 Coll, precision 3%)
                  Measurement very challenging!
                                 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 (β*) optics      Schematic layout of the ALFA
    and reduced beam emittance.
                                             detector in Roman pot
•   The main requirements on the
    tracker are resolution below 100μm
    and sensitive edge region

      ALFA detector final installation and first measurements in 2009.
                       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 |η|>8.3.

•   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
                                                  6 tungsten-quartz calorimeter modules
•   For HI collisions, ZDC will provide low       1 EM and 3 Hadronic
    rate trigger for ultra peripheral
    collisions and will measure collision

  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 σ(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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