Heavy Ion Physics with the ATLAS Detector

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Heavy Ion Physics with the ATLAS Detector

• Heavy Ion Physics working group
• K.Assamagan, B.Cole, J.Dolejsi, F.Gianotti,
  J.Nagle, P.Nevski, A.Olszewski, L.Rosselet,
  H.Takai, S.Tapprogge, S.White, R.Witt, B.Wosiek,
  K.Wozniak

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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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From RHIC to LHC
?sNN 200 GeV 5,500 GeV

• Initial state fully saturated (CGC)
• Enormous increase of high-pT processes over RHIC
  
• Plenty of heavy quarks (b,c)
• Weakly interacting probes become available (Z0,
  W?)

LHC
RHIC
SPS
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ATLAS as a Heavy Ion Detector
1. Excellent Calorimetry

• Hermetic coverage up to ? lt 4.9
• Fine granularity (longitudinal and lateral
  segmentation)
• Very good jet resolution

High pT probes (jets, jet shapes, jet
correlations, ?0)

• Large Acceptance Muon Spectrometer

• Coverage up to ? lt 2.7

Muons from ?, J/?, Z0 decays

• Inner Detector (Si Pixels and SCT)

• Large coverage up to ? lt 2.5
• High granularity pixel and strip detectors
• Good momentum resolution

Tracking particles with pT ? 0.5 GeV/c
2. 3.
Heavy quarks(b), quarkonium suppression(J/? ,?)
1. 3.
Global event characterization (dNch/d?, dET/d?,
flow) Jet quenching
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Physics program

• 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)
• Idea take full advantage of the large
  calorimeter and µ-spectrometer

Direct information from QGP
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Studies of the Detector Performance

• Constraint No modifications to the detector.

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

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

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Central PbPb Collision (blt1fm)
Nch(??0.5)

• 40,000 particles in ? ? 3
• CPU 6 h per central event (800MHz)

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Detector Occupancies
b 0 1fm
Calorimeters ( ?lt 3.2 )
Si detectors Pixels lt 2 SCT lt
20 TRT too high occupancy, not used
for these studies (limited usage for AA
collisions is under investigation) Muon
Chambers 0.3 0.9 hits/chamber (ltlt pp at
1034 cm-2 s-1)
Average ET (uncalibrated) 2
GeV/Tower .3 GeV/Tower
0.1?0.1 Tower
0.1?0.1 Tower
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Global Event Characterization
Day-one measurements Nch, dNch/d?, ?ET,
dET/d?, b

• Constrain model prediction indispensable for
  all physics analyses

Single PbPb event, b 0-1fm
dNch/d??0 ?3200 Points reconstructed HIJING,
no quenching
Histogram true Nch
Points reconstructed Nch
Nch(? lt 3)
measured
Reconstruction errors 5
No track reconstruction used, only Nhits as
calibrated in pp
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Estimate of Collision Centrality
Use 3 detector systems to obtain impact
parameter PixelSCT, EM-Cal HAD-Cal
Resolution of the estimated impact parameter
1fm for all three systems.
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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 gt 70
• fake rate 5
• pT-resolution 3

• 2000 reconstructed tracks per HIJING (b0) event
  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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Heavy Quark Production
Motivation Heavy quarks may radiate less energy
in the dense medium
(dead-cone effect) than light quarks.
b-tagging capabilities offer additional tool to
understand quenching.

• To evaluate b - tagging performance
• pp?WH?l?bb events overlayed
• on HIJING background have been used.
• A displaced vertex in the Inner Detector
• has been searched for.

Rejection factor against u- jets 100 for
b-tagging efficiency of 25
Should be improved by optimized algorithms and
with soft muon tagging in the Muon Spec.
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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.
• Measuring jet profile is the most direct way to
  observe any change.

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Jet Rates
For a 106s run with PbPb at L4?1026 cm-2 s-1
we expect in ? lt 2.5
And also 106 ? jet events with ET gt 50 GeV
500 Z0(??) jets with ET gt 40
GeV
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Jet Reconstruction

• Sliding window algorithm, ?? ? ?? 0.4 ? 0.4,
• after subtracting the pedestal (the average
  pedestal is 50 ?11 GeV)
• Accepted, if ET(window) gt 40 GeV

Di-jet reconstructed in PbPb
Di-jet event from PYTHIA in pp pThard55GeV
Di-jet embedded in PbPb before
pedestal subtraction
Di-jet embedded in PbPb after pedestal subtracti
on
The used algorithm is not optimal, studies are
ongoing.
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Jet reconstruction efficiency
Pb-Pb collisions (b 0 1 fm)
Angular resolution for 70 GeV jets
Efficiency
2 ? resolution in pp
Fake rate
Energy resolution
Pb-Pb
p-p

• For ET gt 75GeV efficiency gt 95, fake lt 5
• Good energy and angular resolution
• Next use tracking information to lower the
  threshold and reduce the fakes

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Jet Studies with Tracks
ETcore Measurements
Fragmentation function
Energy deposited in a narrow cone R lt 0.11
(HADCal), R lt 0.07 (EMCal)

• Jets with ET 100 GeV
• Cone radius of 0.4
• Track pT gt 3 GeV

ltETcoregt sensitive to 10 change in ETjet
The background has not been subtracted
ltETcoregtPbPb ? ltETcoregtpp
PbPb ? HIJING-unquenched ? pp
Promising, but a lot of additional work is needed!
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Quarkonia Suppression
Color screening prevents various ?, ?, ? states
to be formed when T?Tc 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 µ-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 ? lt 1.2 (6 acceff).
J/? ? ??
- a study is under way (?mass 53 MeV).
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Trigger and DAQ

• Interaction rate of 8 kHz for L 1027 cm-2 s 1
• Event size 5 MB for central PbPb (a
  conservative estimate)
• Data rate to storage the same as in pp 300 Hz ?
  MB
• Output rate (after HLT) 50 Hz for central events

• Unbiased interaction trigger (LVL1) use
  forward calorimeters (FCAL)
• - Triggering on the total ET in FCAL with a
  Trigger Tower threshold
• of 0.5 GeV selects 95 of the inelastic
  cross-section.

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Trigger and DAQ

• Triggering on events with b lt 10 fm - use full
  ATLAS Calorimetry

• High Level Triggers (ATLAS T/DAQ) - jet
  trigger, di-muon trigger,...
• Jet rate 40 Hz for ET threshold
  50 GeV
• 0.1 Hz for ET
  threshold 100 GeV

Selection signatures
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Ultra-Peripheral Nuclear Collisions

• High-energy ??-? and ?-nucleon collisions
• Measurements of hadron structure at high
  energies (above HERA)
• Di-jet and heavy quark production
• Tagging of UPC requires a Zero Degree
  Calorimeter
• Ongoing work on ZDC design and integration
  
• with the accelerator instrumentation

Proton-Nucleus Collisions

• Link between pp and AA physics
• Study of the nuclear modification of the gluon
  distribution at low x.
• Study of the jet fragmentation function
  modification
• Full detector capabilities (including TRT) will
  be available.
• L1030 translates to about 1MHz interaction
  rate (compare to 40 MHz in pp)

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Summary

• This work represents a just a first look into
  heavy-ion physics
• with the ATLAS detector.
• The high granularity and large coverage of the
  calorimeter system,
• the acceptance of the muon spectrometer and
  the tracking
• capabilities of the inner detector allow for
  a comprehensive study
• of high-pT phenomena and heavy quark
  production in heavy-ion
• collisions.
• Studies of the detector and physics performance
  are ongoing
• Optimization of algorithms for
  high-multiplicity environment
• The flow effects and its impact on the jet
  reconstruction
• pA collisions

These first results already demonstrate a very
good potential of the ATLAS experiment for
heavy-ion studies and ensure its valuable and
significant contribution to the LHC heavy-ion
physics programme.