Pavel Nevski

ATLAS detector performance in ATLAS detector performance in Heavy Ion Collisions at LHC Heavy Ion Collisions at LHC

Pavel Nevski Pavel Nevski

BNLBNL MotivationsMotivations

Event CharacteristicsEvent Characteristics

Subsystems PerformanceSubsystems Performance

Pavel Nevski

Heavy Ions at the LHCHeavy Ions at the LHC Initial energy density about 5 times higher than at RHIC:

Lifetime of a quark-gluon plasma much longer : 10-15 fm/c at LHC as compared to 1.5-4 fm/c at RHIC Access to truly hard probes with sufficiently high rates : pT > 100 GeV/c (at RHIC pT 20 GeV/c) copious production of b and c quarks deconfinement restoration of the chiral symmetry, physics of parton densities close to saturation

Study of QCD matter at extremely high energy densities and ~vanishing baryon chemical potential:

RHIC LHC 200 5500 GeV

NNs

Pavel Nevski

ATLAS as a Heavy Ion DetectorATLAS as a Heavy Ion Detector1. High Resolution E.M. and Hadronic Calorimeters— Hermetic coverage up to || < 4.9— Fine granularity (with longitudinal segmentation)

2. Large Acceptance Muon Spectrometer— Coverage up to || < 2.7

3. Si Tracker— Large coverage up to || < 2.5— Finely segmented pixel and strip detectors— Good momentum resolution

High pT probes

Muons from , J/, Z0 decays

Tracking particles with pT 1.0 GeV/c2.+ 3. Heavy quarks(b), quarkonium suppression(, ’)

1.& 3. Global event characterization

Pavel Nevski

ATLAS DetectorATLAS Detector

ATLAS is an excellent detector

for high pT physics and jet studies

Pavel Nevski

Simulation DataFlowSimulation DataFlow

Pavel Nevski

Simulation ToolsSimulation Tools: Generators: GeneratorsHIJING Event Generator:

Based on PYTHIA and Lund fragmentation scheme(Soft string dynamics + hard pQCD interactions)

with nuclear effects: nuclear shadowing, jet quenchingHowever, HIJING jet quenching model does not fit the RHIC measurements

quenching, no shadowing

quenching, shadowing

no quenching, shadowing

Pb+Pb b=0 fm sNN=5.5 TeV

Pavel Nevski

Stable Particles after HIJINGStable Particles after HIJINGPer 10 Events

All decays faster then pi0 are now done by hijing,But you can switch some of them off

Pavel Nevski

Central Pb+Pb Central Pb+Pb Collision in ATLASCollision in ATLAS

About 75,000 stable particles ~ 40,000 particles in || 3.2 CPU – 6 h per central event (800MHz) Event size 50MB (without TRT)

Nch(|y|0.5)

Pavel Nevski

Simulated Event SamplesSimulated Event SamplesHIJING + full GEANT3 ATLAS detector simulationsOnly particles within |y| < 3.2 for the moment

High Geant thresholds 1 MeV tracking/10 MeV production

— 5,000 events in each of 5 impact parameter bins: b = 0-1, 1-3, 3-6, 6-10, 10-15 fm

Standard ATLAS thresholds 100 keV tracking/1 MeV production

— 1,000 central events, b = 0-1fm

Initial layout – 2 pixel barrel layers— 1,000 central events, b = 0-1fm

Pavel Nevski

Global MeasurementsGlobal MeasurementsDay-one measurements:

Nch, dNch/d, ET, dET/d, b

Constrain model prediction Indispensable for all physics analyses

Predictions for Pb+Pb central collisions at LHC

(dNch/d)0 Model/data

~12500 HIJING:with quenching, no shadowing ~ 6500 HIJING:with quenching, with shadowing ~ 3200 HIJING:no quenching, no shadowing ~ 2300 Saturation Model (Kharzeev & Nardi) ~ 1500 Extrapolation from lower energy data

Pavel Nevski

Measurements of NMeasurements of Nchch(|(|| < 3)| < 3)

Based on the correlation between measurable quantity

Q and the true number of charged primary particles:

Q = f(Nch)

Q: Nsig- all Si detectors,except

PixB

EtotEM, Etot

HAD

ETEM , ET

HAD

Caution:•Consistency between the measured signals and the simulated ones•Monte Carlo dependency

Pavel Nevski

Estimate of the Collision CentralityEstimate of the Collision CentralityMonotonic relation between measurable quantities Q and centrality parameter b (Npart,Ncoll) allows for assigning to a certain fraction of events, selected by cuts on Q, a well defined average impact parameter. Correlation improves with a larger rapidity coverage.Nsig ET - EM ET - HAD

Pavel Nevski

Event ReconstructionEvent Reconstruction Most of the standard ATLAS reconstruction Most of the standard ATLAS reconstruction

packages developed for PP physics are working on packages developed for PP physics are working on HI events after minimal parameter tuning:HI events after minimal parameter tuning:– We have successfully exercised all calorimeter We have successfully exercised all calorimeter

reconstruction - photons, jets, missing energy.reconstruction - photons, jets, missing energy.– Silicon Pixel and Strip detectors have reasonable Silicon Pixel and Strip detectors have reasonable

occupancy and can provide track reconstruction already occupancy and can provide track reconstruction already with existing PP codes.with existing PP codes.

– Muon reconstruction is even simpler in HI events Muon reconstruction is even simpler in HI events - provided the muon energy is above 6 GeV- provided the muon energy is above 6 GeV

Dedicated HI reconstruction packages will be Dedicated HI reconstruction packages will be developed in due time:developed in due time:– Jet reconstruction is a tricky issue -work is ongoing to Jet reconstruction is a tricky issue -work is ongoing to

develop an appropriate codedevelop an appropriate code

Pavel Nevski

Inner Detector OccupancyInner Detector Occupancy

Pixel Detector Silicon Tracker

Impact parameter b=0-1 fm, HIJING event generator.

TRT is excluded from analysis

Pavel Nevski

Track ReconstructionTrack ReconstructionTrack reconstruction performed with ATLAS pp tracking code using the Pixel and SCT detectors (xKalman++). —pT threshold for reconstructed

tracks is set to 1 GeV.—Tracking cuts are optimized to

get a decent efficiency and

low rate of fake tracks.— Further high pT fake rejection

can be achieved using calorimeter

For pT 1 to 15 GeV/c:efficiency ~ 70 %fake rate ~5 %

Much better in |y|<1May very with cuts:Eff. ~80% , fake rate 15-20%Eff. ~65%, fake rate ~2%- Subject to further optimization

Pavel Nevski

Track Reconstruction Track Reconstruction Momentum resolutionEfficiency versus rapidity

Flat dependency for |y| < 2 ~3% for pT up to 20 GeV/c ~2% for |y|<1

Pavel Nevski

CalorimetryCalorimetryEnergy Per Energy Per

Cell:Cell:

0.10 x 0.10 0.10 x 0.10 cell in e.m. cell in e.m. calorimetercalorimeter

0.10 x 0.10 0.10 x 0.10 cell in hadron cell in hadron calorimetercalorimeter

Pavel Nevski

Jets and Jets and ClustersClusters

Reconstructed e.m. Reconstructed e.m. clusters – exotic clusters – exotic processes can be processes can be observed with cluster observed with cluster energy more than ~15 energy more than ~15 GeV (?)GeV (?)

Reconstructed hadronic Reconstructed hadronic jets – jet signature can jets – jet signature can be used with Pt above be used with Pt above 50 GeV (?)50 GeV (?)

Pavel Nevski

Modified Jet ReconstructionModified Jet Reconstruction

- Pythia jets embedded in Hijing events- Local energy level is evaluated and subtracted- Reconstructed Jet parameters are compared to MC truth for embedded jets

Pavel Nevski

Heavy Quark ProductionHeavy Quark ProductionHeavy quarks live through the thermalization of QGP

can be affected by the presence of QGP Their radiative energy loss is different than for light

quarks.Preliminary study:

—Standard ATLAS algorithm for pp—Higgs events embedded into pp or Pb-Pb event—Cuts on the vertex impact parameter in the Pixel and SCT

Promising, should be improved when combined with muon tagging!

Rejection factors against light quarks versus b-tagging efficiency

p-p Pb-Pb

Pavel Nevski

MuonsMuons On average muons loose 5 GeV in calorimeter On average muons loose 5 GeV in calorimeter

and have strong multiple scattering angles and have strong multiple scattering angles - Use combined info from ID+muon - Use combined info from ID+muon

spectrometer to increase accuracyspectrometer to increase accuracy Association based on Association based on geometrical cuts: geometrical cuts: φ x φ x η after back extrapolation at vertexη after back extrapolation at vertex + + global fitglobal fit of all possible combinations, of all possible combinations,

ordered in decreasing χ2 + ordered in decreasing χ2 + χ2 cutχ2 cut Loose cutsLoose cuts at the beginning: at the beginning: 96.2% of μ from 96.2% of μ from are kept are kept Compare 2 samples: pure Compare 2 samples: pure and Hijing events: and Hijing events: 5000 5000 μ+μ- generated with T=240 MeV μ+μ- generated with T=240 MeV 5000 Pb-Pb Hijing events with b=0 5000 Pb-Pb Hijing events with b=0 ( after full Geant 3 + reconstruction)( after full Geant 3 + reconstruction) Invariant mass is calculated using the

overall fit

Pavel Nevski

Quarkonium SuppressionQuarkonium SuppressionUpsilon family (1s) (2s) (3s) Binding energies (GeV) 1.1 0.54 0.2Dissociation at the temperature ~2.5Tc ~0.9Tc ~0.7Tc

Signal – 5000 generated μ+ μ- μ+ μ- Hijing(b=0): 0.18 0.18 μ , μ , 0.0080.008 μ+ μ- pairs reconstructed per event μ+ μ- pairs reconstructed per event For min.bias -> 0.00090.0009 expected μ+ μ-/ev, 387,640 mixed pairs expected μ+ μ-/ev, 387,640 mixed pairs

8b/410μb 8b/410μb >>-> 18 background pairs per one +– Background estimate (HIJING+G3) S/B ~ 0.6:

Pavel Nevski

Trigger DAQTrigger DAQFor Pb+Pb collisions the interaction rate is 8kHz, a factor of 10 smaller than LVL1 bandwidth.

We expect further reduction to 1kHz by requiring central collisions and pre-scaled minimum bias events (or high pT jets or muons).

The event size for a central collision is ~ 5 Mbytes.

Similar bandwidth to storage as pp at design L implies that we can afford ~ 50 Hz data recording.

Pavel Nevski

ConclusionConclusion

ATLAS detector will be capable of measuring many aspects of High pT Heavy Ion physics

Simulation, Reconstruction and Analysis tools exist to evaluate the detector performance

Work is in progress to understand the detector performance for studying the truly high pT phenomena