Heavy Ion Physics with the ATLAS Detector

May-2003 Barbara Wosiek 1

Heavy Ion Physics with the ATLAS Detector

Barbara [email protected]

Institute of Nuclear Physics, Kraków, Poland

For the ATLAS Heavy Ion Group:

S. Aronson, K. Assamagan, B. Cole, M. Dobbs, J. Dolejsi,H. Gordon, F. Gianotti, S. Kabana, M. Levine, F. Marroquin,J. Nagle, P. Nevski, A. Olszewski, L. Rosselet, P. Sawicki,

H. Takai, S. Tapprogge, A. Trzupek, M.A.B. Vale,S. White, R. Witt, B. Wosiek, K. Woźniak and ……

mailto:[email protected]


Outline of the Talk

INTRODUCTION

Why Heavy Ions at the LHC? Why ATLAS as a Detector for Heavy Ions?

ATLAS PERFORMANCE FOR HEAVY ION PHYSICS

Monte Carlo Simulations Detector Occupancies Global Measurements Event Characterization Tracking with ATLAS ID(Si) B-tagging Quarkonia Studies JET PHYSICS Ketevi’s talk


Heavy Ions at the LHCStudy of QCD matter at extremely high energy densities and ~vanishing baryon chemical potential.

Initial energy density about 5 times higher than at RHIC. Lifetime of a hot & dense matter 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

RHIC LHC 200 5500 GeVNNs


Heavy Ions at the LHC

Quantitative studies of a QGP properties: Hot/dense matter effects should dominate over initial and final state effects. Studies facilitated by many hard probes.

LHC – HI Phase I

Pb + Pb E=2.75 TeV/beam Lmax = 1·1027 cm-2 s-1 Interaction Rate = 8 kHz

Exploratory run of a few days in 2007Extended run in 2008 (1 nb-1)

LHC – HI Phase II and later

• p + A collisions (benchmarking nuclear effects)• Possible lighter ion species: 115 In, 84Kr, 40Ar, 16O


Heavy Ion Physics with ATLAS Detector

ATLAS interest in heavy ion physics was activated by highlights from RHIC experiments!

Hot/dense Nuclear Matter Diagnostics Suppression of high pT particles Disappearance of back-to-back high pT jet correlations Huge azimuthal asymmetry at high pT

ATLAS is an excellent detector for high pT physics and jet studies


ATLAS as a Heavy Ion Detector

1. High Resolution 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 , Z0 decays

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

1.& 3. Global event characterization


Simulation ToolsEvent Generator HIJING :

Based on PYTHIA and Lund fragmentation schemewith nuclear effects: nuclear shadowing, jet quenching

Simulated event samples

HIJING + full GEANT3 ATLAS detector simulationsOnly particles within |y| < 3.2

High Geant cuts: 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 cuts: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


Central Pb+Pb Collision

About 75,000 stable particles ~ 40,000 particles in || 3 CPU – 6 h per central event (800MHz) Event size 50MB (without TRT)

Nch(|y|0.5)


Average Occupancies

Central Collision Events b=0-1 fm

Occupancies still reasonable in all Si Detectors: below 2% in Pixels and below 20% in Strips (after accounting for local fluctuations in the data with low GEANT cuts) TRT unusable – too high occupancy


Global Measurements

DAY-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

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


Measurements of Nch(|| < 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-B)

EtotEM, Etot

HAD

ETEM , ET

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


Measurements of Nch(|| < 3)

Reconstructed multiplicitydistribution (Nsig)

Relative reconstructionerrors: |Nrec-Nch|/Nch

Histogram – true Nch

Points – reconstructed Nch

Uncertainty up to 10% at low Nch, less than 3% at high Nch


Reconstruction of dNch/d

Motivation: shape of the dNch/d distribution is sensitive to dynamical effects like e.g. quenching and shadowing.

Analysis is based on signals only from Pixel barrel layers (done separately for each layer).

Clusterization procedure i.e. merging of hits in neighbor pixels is applied (particle traverses more than one pixel when 0).

Correction factors need to be applied to account for the excess of clusters at large ||:• double hits from overlapping sensors• magnetic field effects (low pT particles bending back)• production of secondary particles


Reconstructed dNch/d

Single Pb+Pb event, b =0-1fm 5 peripheral collision, b =10-15fm

Correction factors are ~ centrality independent!

Comparison of the reconstructed dNch/d distributions

with the true one of charged primary particles.One single correction function C() calculated from a sample of central events is used.

Reconstruction errors ~5% Reconstruction errors ~13%


Reconstructed dNch/d

Single Pb+Pb HIJING eventwith jet quenching, b =0-1fm

100 p+p events at s=200 GeV

Correction factors are ~insensitive to the detailedproperties of generated particles!

Different shape and higher density are correctly reproduced!!


Estimate of the Collision Centrality

Based on the correlation between the measurable quantity Q and the centrality parameter: b, (Npart, Ncoll)

Monotonic relation between Q and b allows for assigning to a certain fraction of events selected by cuts on Q,

a well defined average impact parameter.

Nsig ET - EM ET - HAD


Estimate of the Collision CentralityResolution of the estimated impact parameter

Remark: A better approach would be to use a quantity measured outside the mid-rapidity region,e.g. energy in forward calorimeters, which is less sensitive to dynamical effects.


Track Reconstruction

Pixel and SCT detectors – ATLAS xKalman algorithmStarting with software release 6.1.0 all modifications specific to Heavy Ion track reconstruction are included in the official xKalman code.— pT threshold for

reconstructable

tracks is 1 GeV (reduce CPU).—Tracking cuts are optimized to

get a decent efficiency and

low rate of fake tracks.•At least 10 measurements per track•Maximum two shared measurements2/ndf 4

• Tracking in the || < 2.5

• For pT: 1 - 15 GeV/c: efficiency ~ 70; fake rate<10%• Fake rate at high pT can be reduced by matching with Calo data


Track Reconstruction Momentum resolutionEfficiency versus rapidity

Flat dependency for |y| < 2Higher in EC (more layers)

~3% for pT up to 20 GeV/c(for || < 2.5)

Tracking in HI events looks promising, still can be optimized!


Heavy-Quark Production

Heavy quarks live through the thermalization of QGP can be affected by the presence of QGP Their radiative energy loss is qualitatively different than for light quarks.

Open Beauty via semi-leptonic decays

Tagging of the B-jets:

— the high pT in the MS

— displaced vertex in the ID


B-jet TaggingPreliminary 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


Quarkonium Suppression

Upsilon mass reconstruction using the Muon Spectrometer, Silicon Tracker and the Pixel Detector (barrel sections only).

Direct probe of the QGP:Color screening of the binding potential leads to the

dissociation of the quarkonium states.

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

Important to separate (1s) and (2s)

+–


Quarkonium Suppression— GEANT3 simulations of pure (1s) and (2s) states +–

— Muons with pT > 3GeV are tracked backwards to the ID — Invariant mass is calculated from the overall fit.

— Background estimate (HIJING+G3) S/B ~ 0.6— Acceptance 10-15% providing 100% efficient dimuon trigger — Overlay with HIJING Event is under study!

= 130 MeV


Summary

ATLAS detector will be capable of measuring many aspects of relatively low pT heavy-ion physics

Let’s see the detector performance for studying the truly high pT phenomena …


BACKUPS


Detector Occupancies

Occ Occ

zzNch Nch

Examples of occupancy versus z and Nch(high GEANT thresholds)

Pix1 SCT1


Detector occupancies

Pixel Detector Silicon Tracker

Central collisions b=0-1 fm, low GEANT thresholds


Trigger DAQ

For Pb+Pb collisions the interaction rate is 8kHz, a factor of 10 smaller than LVL 1 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.

~200 Hz


Correction Factors

truech

cluster

)ηd/dN(

)ηd/dN()η(C

Correction factors are defined as:

C() calculated from the sample of 50 central(b=0-1fm) Pb+Pb events,and then parameterized.

Correction function forthe inner most barrel

layer.


Maximal Cluster Size

Define the expected maximal size of the cluster: In Z-direction the number of pixels to be merged depends on the Z-coordinate of the hit:

Rd

m300)ZZ(

N Sivtxhitpixels

e.g. for R=5cm, Zhit=40cm Npixel 6 -7

In -direction the number of traversed pixels depends on pT.

For a track with a curvature r, an angle at which particle enters the sensor is cos()=R/2r (assuming that sensors form an ideal tube). Taking r = 15cm (corresponding to pT=90 MeV/c):

Npixels = 4 – 6 for R=12cmNpixels = 3 – 4 for R= 5cm


Cluster Formation

Choose seeds large signals > 10,000 electrons

Start with the seed with the largest signal

Attached to it a signal in the adjacent pixel as long as:

There is a signal in a pixel One of the closest neighbor pixels already belongs to the cluster The distance from the seed to the pixel is not larger than the expected maximal size of the cluster (in both Z and directions)up to 6 pixels in Z (depending on Zhit) and 3 pixels in (depending on R)

Heavy Ion Physics with the ATLAS Detector

Documents

Transcript of Heavy Ion Physics with the ATLAS Detector