Particle multiplicity & ET measurements with ATLAS

Andrzej OlszewskiInstitute 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 ……

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Outline of the Talk

INTRODUCTION

Global observables in experiments at LHC Global measurements in Atlas detector

ATLAS MEASUREMENTS OF Nch and ET Monte Carlo Simulations Detector Occupancies Global Measurements Event Characterization Tracking with ATLAS ID(Si)

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Heavy Ions at the LHC

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

deconfinement restoration of the chiral symmetry, physics of parton densities close to saturation

Hot/dense matter effects should dominate over initial and final state effects. The effects may be seen already in global observables like particle multiplicity and ET

Quantitative studies of a QGP properties:

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The ATLAS Detector

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ATLAS as a Heavy Ion Detector

2. High Resolution Calorimeters— Hermetic coverage up to || < 4.9— Fine granularity (with longitudinal segmentation)

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

Global event characterizationTracking particles with pT 1.0 GeV/cHigh pT probes, heavy quarks, quarkonia ...

1. Si Tracker— Large coverage up to || < 2.5— Finely segmented pixel and strip

detectors— Good momentum resolution

Physics:

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Heavy Ion Interactions in Atlas

AA

collis

ion

Geometry of a collision

Binary NN approximation

Soft & Hard processes

Parton distributions

Nuclear modifications

Nch,Etot,ET

Dete

cto

r

Number of signals in Si Tracker

ET in EM and HADcalorimeters

Etot in EM and HADcalorimeters

Number of tracksreconstructed

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Simulation Tools

Event generator HIJING Based on PYTHIA and Lund fragmentation scheme with nuclear effects: nuclear shadowing, jet quenching

GEANT3 detector simulation Full GEANT3 ATLAS detector simulations High Geant cuts: 1 MeV tracking/10 MeV production Only particles within |y| < 3.2

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

Detectors used in analysis Silicon Pixel, SCT. EM and HAD Calorimeters

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Zoom on Atlas ID & Calorimeters

Hadronic TileCalorimeters

EM AccordionCalorimeters

Silicon Tracker in Inner Detector

Forw

ard

LAr

Calo

rimete

rsH

adro

nic

LA

r End C

ap

Calo

rim

ete

rs

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Central Collision Events b=0-1 fm

Average Occupancies

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

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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

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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

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Calibration

4.1NDF/,Ne3.8N85.34337N 22ch

05chhits

<Nhits> vs. <Nch> <Nhits> vs. <Nch>

Zoom

Linear calibration:

7NDF/,N1158.3327.5425N 2chhits

Quadratic calibration:

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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

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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

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dNhits/d vs. dNch/d

dN

hit

s/d

R= 5cmR= 9cmR=12cm

dNch/dtrue-primary

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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

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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%

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Reconstructed dNch/d

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

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

100 p+p events at s=200 GeV

Different shape and higher density are correctly reproduced!!

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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

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Stability of the Centrality Estimate

Comparison of centrality cuts based on Nsig, Etot, ET

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.

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Resolution of the Centrality Estimate

Resolution as a functionof cut width

Loss of resolution relative to pure b cuts

High resolution available by using narrow cutson centrality correlated quantity

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Track Reconstruction

Pixel and SCT detectors – ATLAS xKalman algorithm

—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.—Vertex constraints applied

—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 calorimeter

—At least 10 measurements per track—Maximum two shared measurements2/ndf 4

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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!

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Summary

Global measurements are needed for detailed studies of heavy ion interaction properties

ATLAS detector is capable of providing measurements of the total charged multiplicity and transverse energy as well as their rapidity densities using very simple reconstruction procedures

Precision of the measurements seems to be high and reasonably independent of the true collision properties

These results, available already in the first days of LHC Heavy Ion run, may provide on their own crude verification of some ideas in heavy ion models

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BACKUPS

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Zoom on Atlas ID & Calorimeters

Hadronic TileCalorimeters

EM AccordionCalorimeters

Silicon Tracker in Inner Detector

Forw

ard

LA

rC

alo

rimete

rsH

ad

ron

ic L

Ar

En

d C

ap

Calo

rim

ete

rs

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Detector Occupancies

Occ Occ

zzNch Nch

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

Pix1 SCT1

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Detector occupancies

Pixel Detector Silicon Tracker

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

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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

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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.

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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)