Heavy Ion Physics with the ATLAS Detector for the ATLAS Collaboration

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Heavy Ion Physics with the ATLAS
Detectorforthe ATLAS Collaboration
Sebastian White Brookhaven National Lab
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Jet Physics with ATLAS
Strikman,Vogt SNW
-Gyulassy Vitev
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Studies of the Detector Performance

• Using existing ATLAS detector and (mostly)
  algorithms
• developed for pp _at_L1034 (24 interactions _at_40Mhz)

• Simulations HIJING event generator, dNch/dh
  3200
• Full GEANT
  simulations of the detector response

• Large event samples
• hlt 3.2 impact parameter range b 0
  - 15fm (27,000 events)
• hlt 5.1 impact parameter range b
  10 - 30fm (5,000 events)

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Simulation DataFlow
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ATLAS Inner Tracking (to hlt2.5 )
PixelSCT11 layers(2-38) TRT39 layers
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Tracking Reconstruction
Full ATLAS Simulation
-2.5lthlt2.5
Tile Calorimeter threshold cut will have
excellent rejection.
Detailed reconstruction with pTthr 300 MeV/c
Track 10 of 11 planes. Most fakes in forward
directions.
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Tracking Resolution
Full ATLAS Simulation
Plot shows the average reconstruction resolution.
dpt/pt2 in barrel, larger in endcaps
-2.5lthlt2.5
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State of the art EMHad calorimeter! -Highly
segmented. -4 depth segments -200k EM
channels. -LAr to hlt4.9
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EM Calorimeter Segmentation
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Jet Reconstruction
Energy in 0.1x0.1 tower in the EM and HAD
calorimeter for hlt3.2. Most of the energy is in
the EM calorimeter due to soft particles ranging
out. Hadronic calorimeter is relatively quiet
even in b0 HIJING events!
Energy in 0.1x0.1 tower as function of h.
Full ATLAS Simulation
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55 GeV Jet
PYTHIA HIJING overlayed event.
PYTHIA only
After average background subtraction
For lower jet energies we found that it is easier
to find jets if the first EM compartment is not
included. Later the energy from the 1st
compartment is recovered.
The Result !
Full ATLAS Simulation
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Jet Reconstruction Efficiency
Full ATLAS Simulation
Very promising results with high jet
reconstruction efficiency!
A good jet is defined as the one that finds a
match in the generated event within a cone radius
of 0.2. Fakes are the ones that do not fulfill
the requirement. Fakes include HIJING jets. Track
matching may reduce the number of real fakes.
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Jet Reconstruction
sh,f.045,.035
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Jet Energy Resolution
Excellent jet energy resolution. Energy
resolution is close to a high luminosity L1034
proton-proton run. This fact also means that
large contingent of high energy ATLAS
participants are interested in working on these
issues.
Full ATLAS Simulation
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Weight7000T diameter22m Width44m
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Quarkonia Suppression
Color screening prevents formation of various ?
states when TgtTc for the phase transition to QGP
(color screening length lt size of
resonance)
QGP thermometer
Upsilon family
?(1s) ?(2s) ?(3s) Binding energies
(GeV) 1.1
0.54 0.2 Dissociation at the
temperature 2.5Tc 0.9Tc
0.7Tc
Important to separate ?(1s) and ?(2s)!
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Upsilon Reconstruction
? ? ??

• Overlay ? decays on top of HIJING events.
• Use combined info from ID and m-Spectrometer

• Single Upsilons
• HIJING background
• Half ?s from c, b decays, half from p, K decays
  for pTgt3 GeV.
• Background rejection
• ?2 cut
• geometrical ?? ? ?? cut
• pT cut.

??,?? differences between ID and µ-spectrometer
tracks after back-extrapolation to
the vertex for the best ?2 association.
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Upsilon Reconstruction
Barrel only (? lt1)
A compromise has to be found between acceptance
and mass resolution to clearly separate upsilon
states.
? lt1 ?
lt2.5 Acceptance 4.9 14.1
efficiency Resolution 123 MeV 147
MeV S/B 1.3
0.5 Purity 94-99 91-95
For a 106s run with PbPb at L4?1026 cm-2 s-1
we expect 104 events in h lt 1.2 (6 acceff).
J/? ? ??
- a study is under way (?mass 53 MeV).
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Diffractive Physics with ATLAS

• ATLAS Coverage
• Forward Instrumentation
• ATLAS reach in jj and gj

Pro-E model of ZDC for ATLAS and full simulation
of Energy response
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Lessons from Run IVPHENIX J/y and high mass ee-
Photoproduction
PHENIX implemented trigger for small cross
section UPC physics 10 M events collected
(seeG.Baur et al. Nucl-th/03070310)
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Probing small x structure in the Nucleus with
gN-gtjets, heavy flavor.
di-jet photoproduction-gt parton
distributions,x2 by g with momentum fraction,
x1 4pt2/sx1x2 ltygt -1/2ln(x1/x2) Signature
rapidity gap in g direction(FCAL veto)
x1
x2
ATLAS coverage to hlt5 units. Pt 2
Gev rapidity gap threshold
Analogous upc interactions and gap structure
diffractive
Non-diffractive
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Event yields from a 1 month HI (Pb-Pb) run at
Nominal Luminosity. Counts per bin of dpt2
GeV dx2/x2/- 0.25 M.Strikman, R.Vogt, SNW
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Collision Hall at start of installation( Feb. 04)
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Location,location,location
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US-ATLAS Heavy Ion Group S. Aronson, K.
Assamagan, M.Baker,H. Gordon, M. Levine, P.
Nevski, H. Takai, P.Steinberg, S.
White Brookhaven National Laboratory J.
Nagle-University of Colorado B. Cole-Columbia
University Bern, CERN, Geneva,
Prague,Cracow -LOI submitted to LHCC by the
full ATLAS collaboration in May 04 -Broad range
of physics and simulation activities now under
way Atlas.ch/standardmodel/HeavyIons
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280 GeV event
Preliminary efficiency numbers show that jet
reconstruction efficiency is larger than 90
above 50 GeV. Below 50 GeV the efficiency lowers
to approximately 75 with an increase in the
number of ghosts. Remember we are using b0
HIJING events as our test case.
Full ATLAS Simulation
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Motivation High-pT Results from RHIC
Jet quenching observed in AuAu as predicted by
pQCD (unquenching in dAu)
Hard processes excellent probes to test QCD!
PRL91, (2003)
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Heavy ion Physics with the ATLAS
Detector -Introductory- we come from a program at
RHIC which has been productive beyond our
expectations. Much of the physics has been driven
by experimental opportunities due to large
multi-purpose detectors. Key experimental tools
have been excellent tracking coverage with
particle identification, powerful calorimetry
(electromagnetic in the central
region-PHENIX,STAR EMC Hadronic in the forward
region ZDC/SMD and Pcal in the forward region).
Many aspects of Heavy Ion physics evolving now
heavy flavors, UPC, jet studies at the highest pt.