The HeavyIon Physics Programme with the ATLAS Detector

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The Heavy-Ion Physics Programme with the ATLAS
Detector new results from WA98

• Laurent Rosselet

Cartigny, September 14th 2007
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Idea study of the Quark-Gluon Plasma
At ?s 200 GeV/A , QGP is unexpectedly a perfect
fluid with no viscosity PbPb at LHC ?s 5.5
TeV/A ? 1148 TeV for central collisions
T500 MeV, energy
density30 GeV/fm3
cf LQCD prediction for
the transition 1 GeV/fm3 Letter of Intent in
2004 Physics Performance Report for 2007
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ATLAS Heavy-ion physics programme

• Global variable measurement
• dN/d? dET/d? elliptic flow
• azimuthal distributions
• Jet measurement and jet quenching
• Quarkonia suppression
• ? J/? ?c
• p-A physics
• Ultra-Peripheral Collisions (UPC)
• Idea take full advantage of the large
  calorimeter and µ-spectrometer
• A Zero Degree Calorimeter is being added for
  trigger and UPC tagging

TAN region, z140m, on each side
Direct information from QGP
2x4 modules of tungsten/quartz sandwich
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Central Pb-Pb collisions

• Simulation HIJINGGEANT
• dNch/d?max 3200 in central Pb-Pb
• c.f. 1200 from RHIC extrapolation

0.1?0.1 Tower (?Fx??)
0.1?0.1 Tower
??? ? 0.5

• Large bulk of low pT particles is stopped in the
  first layer of the EM calorimeter (60 of
  energy)
• µ-spectrometer occupancy in Pb-Pb lt high-L p-p

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Track reconstruction

• Only Pixel and SCT detectors
• At least 10 hits out of 11 per track
• At most 1 shared hits
• For pT 1 - 10 GeV/c
• efficiency 60-70
• fake rate lt 1
• pT-resolution 3

?lt1

• 2000 reconstructed tracks from HIJING (b0)
  events with pT gt 1 GeV
• 
  and
  ? lt 2.5
• Fake rate at high pT can be reduced by matching
  with calorimeter data
• TRT not considered for this study. Expected to
  be partially (fully) usable in central
  (peripheral) Pb collisions gt electron
  identification

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Heavy quarkonia suppression
Original idea color screening prevents various
?, ?, ? states to be formed , when T?Ttrans
to QGP (color screening length lt size of
resonance)
Modification of the potential can be studied by a
systematic measurement of heavy quarkonia states
characterized by different binding energies and
dissociation temperatures thermometer for the
plasma
1.10 0.74 0.15 2.31
1.13 0.93 0.83
0.74
In fact complex interplay between suppression
and regeneration
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For ? lt 2 (12.5 acceff) we expect 15K ?
???-/month of 106s
For ? lt2.5, pT? gt1.5 GeV we expect 100K
J/????-/month
4 different strategies have been investigated
http//dpnc.unige.ch/users/ros/quarkoniav3.ps A
low pT di-muon trigger is under study (with a
muon pTgt1.5 GeV) Studies of J/??ee- and ??ee-,
of ?c decaying into J/?, of open heavy flavors
are under way
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Jet quenching
Energy loss of fast partons by excitation and
gluon radiation ,
larger in QGP

• Suppression of high-z hadrons and increase of
  soft hadrons in jets.
• Induced gluon radiation results in the
  modification of jet properties like
• 
  a broader angular distribution.
• Conical structure which may result from Cherenkov
  radiation or shock-waves from partons traversing
  the medium (Mach cone).
• Effective suppression of the jet cross section
  within a fixed cone size. Measuring jet profile
  is the most direct way to observe any change.
• Advantage of LHC over RHIC full jets with large
  rates, di-jets, ?-jets, Z0-jets, b-jets.

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Jet studies
PYTHIA jets embedded with central PbPb HIJING
events Main task separation of jets from backgr.
Several jet algorithms and methods of
subtraction are tested (average and local)
Cone algorithm Fast kT jet finder Jet
reconstruction fitting algorithm with first
radial moment Fragmentation functions using ID
tracks Di-jets b-tagged jets ?jet Z0jet
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Jet studies (II)
Jet position resolution (R0.4)
Standard ATLAS solution -cone algorithm - is
intensively studied with different samples
Jet energy resolution
Jet finding energy measurement work for ET gt 40
GeV (15 GeV in pp)
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Summary

• Global observables, including elliptic flow,
  should be accessible from
• day-one, even with a very low luminosity
  (early scheme)
• Jet physics (jet quenching) is very promising,
• jet reconstruction is possible
  despite the additional background
• study of di-jet, ?-jet, Z0-jet
  correlations
• possibility to study separately light
  and heavy q-jets
  
  
  
• Heavy-quarkonia physics (suppression in dense
  matter) well accessible,
• capability to measure and separate ?
  and ?,
• to measure the J/? using a specially
  developed ? tagging method
• A study of ?, J/? ? ee- and of open heavy
  flavor prod. is under way
• Low-x physics and UPC will also be accessible

Laurent Rosselet, HEP 2007, Manchester, July 20th
2007
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ATLAS HI Physics Group Brookhaven National
Laboratory, USA Charles University, Prague, Czech
Republic Columbia University, Nevis Laboratories,
USA University of Geneva, Switzerland IHEP,
Protvino, Russia IFJ PAN, Krakow, Poland Iowa
State University, USA PUC, Santiago, Chile JINR,
Dubna, Russia MePHI, Moscow, Russia Chemistry
Department, Stony Brook University, USA Yale
University, USA
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New result from WA98
First observation of a large pT particle (?0)
suppression at SPS energy in central heavy-ion
collisions (related to jet quenching)
Submitted to Physical Review Letters this month