Capabilities for Heavyion Physics in ATLAS

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Capabilities for Heavy-ion Physics in ATLAS

• Laurent Rosselet

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The ATLAS detector
Length 44m Height 22m
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Some striking features Designed for p-p
L1034 cm-2 s-1 Hermetic calorimeter ?lt4.9
10 units of rapidity!
Electromagnetichadronic
Fine
granularity ???F0.025x0.025 (e.g.) EM

???F0.1x0.1 hadronic
with longitudinal segmentation (3
layers both in EM and hadronic) Large acceptance
µ-spectrometer ?lt2.7 Silicon Tracker ?lt2.5
Finely
segmented pixel and strip detector (SCT)
Good
momentum resolution The Atlas detector is well
suited for high pT physics.
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ATLAS heavy-ion study group

• S. Aronson, K. Assamagan, B. Cole, M. Dobbs,
  J. Dolejsi, H. Gordon, F. Gianotti, S.
  Kabana, S.Kelly, M. Levine, F. Marroquin, J.
  Nagle, P. Nevski, A.Olszewski, L. Rosselet, H.
  Takai, S. Tapprogge, A. Trzupek,
  M.A.B. Vale, S. White, R. Witt, B. Wosiek, K.
  Wozniak and

Constraint no modification to the apparatus,
except software triggers and
peripheral instrumentation.
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Physics

• Global variable measurement
• dN/d? dET/d? elliptic flow
• azimuthal distributions
• Jet measurement and jet quenching
• Quarkonia suppression
• J/? ?
• P-A physics
• Ultra-Peripheral Collisions (UPC)

Direct information from QGP
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Inner detector performances
Occupation Pixels lt2 with HIJING (b0-1
fm) SCT
lt20 TRT unusable too
high occupancy gt 11
hits/track at most
Nch from Nsig, on event-by-event basis with 2
accuracy for central event with 10 for
peripheral
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dNch/d? and charged particle multiplicity
distribution
single Pb-Pb event b0-1 fm error 5
generated vs estimated from number of hits
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Estimate of collision centrality
Monotonic relation between number of hits in the
Pixel detector and b
Accuracy on the determination of b with 3
distinct techniques
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Central HIJING event (b0)
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Track reconstruction

• Only Pixel and SCT detectors
• At least 10 hits out of 11 per track
• At most 2 shared hits
• For pT 1 - 15 GeV/c
• efficiency gt 70
• fake rate 5-10
• pT-resolution 3

• 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

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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
  hadrons in jets.

• Induced gluon radiation results in the
  modification of jet properties like
• 
  a broader angular distribution.
• Could manifest itself as an increase in the jet
  cone size or an effective suppression of the jet
  cross section within a fixed cone size.
• Jet profile measurement would be the most direct
  way to observe any change.
• Also ET, pT imbalance and non-coplanarity between
  jets, 3-jet events.

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Experimental evidence from RHIC
Suppression factor
PHENIX
RAAYieldAu-Au/Yieldp-pltNbinarygtAu-Au
Binary scaling
Nucl.-ex/10304038
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Jets Pb-Pb Hijing (b0-1 fm)
3.2 lt ? lt 3.2

• Background 2 GeV per 0.1x0.1 cell in EM
• 0.2 GeV per tower in HAD
• Soft hadrons completely stop in EM
• Largest background in 1st layer
• 20 GeV in a cone Rv?Fx?? 0.4
• gt fluctuations
• gt threshold for jet reconstruction 30 GeV
• 
  in calorimeters
• cf p-p 15 GeV
• Reconstruction sliding window algorithm
• with
  splitting/merging
• after background energy
  subtraction
• 
  (average and local)
• algorithm is not fully optimized
  yet

0.1x0.1 cell
0.1x0.1 tower
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55 GeV jet PYTHIAHIJING event
PYTHIA Jets
PYTHIA Jets Pb-Pb
JetsPb-Pb bckgd subtr.
Pb-Pb Found Jets
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280 GeV jet PYTHIAHIJING event
PYTHIA Jets
PYTHIA Jets Pb-Pb
Jets Pb-Pb, bckgd subtr.
Pb-Pb Founds Jets
Fake jet from background
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Efficiency

• Good jet if matches generated jet within R0.2
• Jets in HIJING events counts as fakes
• Next use tracking information to
• lower the threshold
• reduce the fakes

For ETgt75 GeV efficiencygt95, fakelt5 ,
excellent energy resolution
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Jet angular resolution
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Jet rate/month
ATLAS accepted jets for central Pb-Pb Jet pT gt
50 GeV 30 million ! Jet pT gt 100 GeV
1.5 million Jet pT gt 150 GeV 190,000 Jet pT gt
200 GeV 44,000
Vitev - extrapolated to Pb-Pb

• Every accepted jet event is an accepted jet-jet
  event since ATLAS has nearly complete phase space
  coverage !
• ?-jet 106 events/month with pT gt50 GeV
• ? and Z0 have no radiation !
• ?-jet 10,000 events/month with pT gt50 GeV with
  ? ?µ µ-
• Z0-jet 500 events/month with pT gt40 GeV
  with Z0 ?µ µ-

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Heavy-quark production

• Radiative energy loss is qualitatively different
  for heavy/light quarks.
• Finite velocity of heavy-q gt less en. loss,
  suppression of colinear g
• Tagging of b-jets
• with a high- pT µ (B?Dµ?)
• with displaced vertices
• 200000 b-jets/month with pT gt 40 GeV

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b-tagged jets
1st attempt based on impact parameter
cuts Rejection factors against light quarks vs
b-tagging efficiency
High-L p-p
Central Pb-Pb
Rejection
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Efficiency
Should be improved when combined with µ tagging
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Quarkonia suppression
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
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Upsilon reconstruction
?? µ µ-

• Study the in a full simulation
  (GEANT3reconstruction)
• µ-spectrometer occupation in Pb-Pb lt high-L p-p
• Upsilon family
  ?(1s) ?(2s) ?(3s)
• Mass (GeV)
  9.460 10.023 10.355
  
• Binding energies (GeV) 1.1
  0.54 0.2
• Dissociation at the temperature
  2.5Ttrans 0.9Ttrans 0.7Ttrans
• gtImportant to separate ?(1s)
  and ?(2s)
• µ µ- mass resolution is only 460 MeV at ? peak
  in the µ-spectrometer gt uses combined info
  from ID and µ-spectrometer in a global fit

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Single Upsilons
??, ?Fdifference between ID and µ-spectrometer
tracks after back-extrapolation to the vertex for
the best ?2 association.
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Single Upsilons HIJING background Half µs
from c, b decays, half from p, K decays for pTgt3
GeV. Background rejection based on ?2 cut,
geometrical cut and pT cut.
??, ?Fdifference between ID and µ-spectrometer
tracks after back-extrapolation to the vertex for
the best ?2 association.
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Resolutionacceptance/efficiency
Cut on the decay µs
A compromise has to be found between acceptance
and resolution to clearly separate ? states with
maximum statistics (e.g. 10 accept.)
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Barrel only (?lt1)
Separation between ? and ? for ?lt1 Acceptance
8.7 (cf full 22.0) efficiency Resolut
ion 126 MeV (152 MeV) S/B
2.0 (0.9) Purity
94-99 (91-95) without ?
contamination in the ? peak
depends on the cuts
Depends on cuts
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• A di-muon trigger using a µ pT cut lt 4 GeV is
  being investigated.

• A J/? study is also under way.
• smass53 MeV gteasy separation of J/? and ?
• Low mass gtdecay µs need an extra pT from the
  J/? or a Lorentz boost to get through the
  calorimeters.
• gtfull pT analysis possible only
  forward and backward where the background is
  maximum.

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p-A physics

• link between p-p and A-A physics
• Study of the modification of the gluon
  distribution in the nucleus
• at low xF
• Study of the modification of jet fragmentation
• pQCD in nuclear environment
• xg(x) enhanced by A1/3 6 in Pb compared to p
• Kinematical access xF gt10-5
• Occupancy in p-Pb as in p-p with 23 pile-up
  events
• Hermetic calorimeter good for asymmetric
  collisions
  
  
• gt ATLAS an excellent p-A detector
• p-Pb L 1030 cm-2 s-1 1 MHz

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Ultra-Peripheral Collisions (UPC)
b gt 2R only electromagnetic
interactions ?? ?N ??
with/without nucleus diffraction ?W
s(??)Z4 W ?? lt 2?hc/RA 200
GeV for Pb s(??)Z4 W ??
lt 2?hc/RA 200 GeV for Pb
F
?
F
?
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Summary

• ATLAS has an excellent calorimeter/muon-spectrome
  ter coverage
• suitable for high-pT heavy-ions physics
• ? physics is accessible
• jet physics (jet quenching) is very promising
• PixelSCT work in Pb-Pb collisions
• (tracking, particle multiplicities
  from hits)
• also p-A, Ultra-Peripheral Collisions (easier
  than Pb-Pb collisions)
• will be studied
•