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Jets in Heavy Ion Collisions at the LHC

Andreas Morsch

CERN

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Outline

What are the new opportunities but also experimental challenges of jet physics on Heavy Ion Collisions ?

How can jets be reconstructed in the high multiplicity heavy ion events ?

How can we observe modifications of the jet structure and use them as a tool to test the medium ?

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Jets in nucleus-nucleus collisions

Jets are the manifestation of high-pT partons produced in a hard collisions in the initial state of the nucleus-nucleus collision.

These partons undergo multiple interaction inside the collision region prior to fragmentation and hadronisation.

In particular they loose energy through medium induced gluon radiation and this so called “jet quenching” has been suggested to behave very differently in cold nuclear matter and in QGP.

Simplistically: Jet(E) →Jet(E-E) + soft gluons (E)

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Medium induced parton energy loss

clogarithmior const

tcoefficientransport ),(ˆ

E

2

E(E)

mEfLqCE qRs

Medium characterized bytransport coefficient:

ˆ q 2

ndensity

GeV 5020

100~/

fm/GeV 1 q̂ QGPfor whereasfm,/GeV 05.0ˆmatter nuclear Cold 22

LHC

QGPcold

cold

ΔE

EΔE

q

Example: BDMPS

Coherent sum over scatterings with free path length and mean qT transfer

Expect large effects !Needs large range of E to measure E(E)

Baier, Dokshitzer, Mueller, Peigne, Schiff (1996); Zakharov (1997); Wiedemann (2000); Gyulassy, Levai, Vitev (2000); Wang ...

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Consequences for the jet structure

pp AA

Decrease of leading particle pT

Increased mult. of low-pT Particles from radiation.

Increase of pT rel. to jet-axisEnergy outside jet cone

Dijet energy imbalanceand acoplanarity

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But also background from underlying event …

… and this has important consequences for Jet identification Jet energy reconstruction

Resolution Bias

Low-pT background for the jet structure observables

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Jets at RHIC

In central Au-Au collisions standard jet reconstruction algorithms fail due to the large energy from the underlying event (125 GeV in R< 0.7) and the relatively low accessible jet energies (< 20 GeV).

Use leading particles very successfully as a probe.

p+p @ s = 200 GeV STAR Au+Au @ sNN = 200 GeV

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RHIC: Jet studies with leading particles

STARSTAR

Phys. Rev. Lett. 91, 072304 (2003).

Pedestal&flow subtracted

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

In central Au+Au Strong suppression of inclusive hadron yield in

Au-Au collisions Disappearance of away-side jet

No suppression in d+Au Hence suppression is final state effect.

Suppression of inclusive hadron yield

Disappearance of away-side correlations

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Sensitivity to transport coefficient

RHIC measurements are consistent with pQCD-based energy loss simulations. However, they provide only a lower bound to the initial color charge density.

Eskola et al., hep-ph/0406319

RAA~0.2-0.3 for broad range of

Surface emission bias limitssensitivity to

q̂

q̂

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Bias from the production spectrum

Strong bias on fragmentation function … which we want to measure

But also low efficiency since only tail is relevant.

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)/1()/()(

n

EEPEpPpEP npartonpartonpartonLLeadingparton

Mean value shifts to pLeading/Eparton =0.6

pLeading [GeV]

100 GeV Jet

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Advantages of reconstructed jets

Since more of the original parton energy is collected: Reduced Surface bias Reduced bias on parton energy

Makes measurement of the fragmentation function possible

Possibility to observe directly the quenched jet and the particles from gluon radiation.

Increases statistics at high ET

Increased sensitivity to medium parameters

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Jet structure observables

hadron

jet

p

E

zln)

1ln( ddN /

ddpdT

ddpdpR

TppJetAA

TAAJet

TAAJet /

/)(

2

2

Longitudinal Structure Transverse Structure

Sensitive to out-of-cone radiation.

)1

ln()ln(zp

E

Salgado, Wiedemann, Phys. Rev. Lett. 93: 042301 (2004) Borghini,Wiedemann, hep-ph/0506218

I. Lokhtin

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Direct measurement of

2 ˆ( , )T q Edy yq

J. Casalderrey-Solana and XNW, arXiv:0705.1352 [hep-ph].

q̂

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Jet physics at LHC: Rates

Jet rates are high at energies at which they can be reconstructed over the large background from the underlying event.

Reach to about 200 GeV Provides lever arm to measure the

energy dependence of the medium induced energy loss

104 jets needed to study fragmentation function in the z > 0.8 region.

A. Accardi et al., hep-ph/0310274 CERN TH Yellow Report

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Jet physics at LHC: New challenges

More than one jet ET> 20 GeV per event More than one particle pT > 7 GeV per event 1.9 TeV in cone of R = 2+2 < 1 ! (*) We want to measure modification of leading

hadron and the hadrons from the radiated energy. Small S/B where the effect of the radiated energy should be visible: Low z Low jT Large distance from the jet axis

Experiments need low- and high-pT capabilities for unbiased jet energy measurements and observation of low-pT hadrons from the gluon radiation.

* For dN/dy = 5000.

UnquenchedQuenched (AliPythia)Quenched (Pyquen)

pT < 2 GeV

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Jet reconstruction in Heavy Ion Collisions

How to reconstructs jets above a large fluctuation background (EBg) ? Restrict identification and reconstruction to domain in which

Emeas >> EBg

Cone size R < 1 pT-cut

Also in this case there is a bias due to the input spectrum Identified jets are on average more collimated.

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Optimal cone size

Jets reconstructed from charged particles:

Need reduced cone sizes and transverse momentum cut !

Ene

rgy

cont

aine

d in

sub

-co

ne R

E ~ R2

Jet Finders for AA do not work with the standard cone size used for pp (R = 0.7-1).R and pT cut have to be optimized according to the background conditions.

Background reduced by 0.42 = 0.16 but 88% of signal preserved.

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Background fluctuations Background fluctuations limit the energy resolution. Fluctuations caused by event-by-event variations of the impact parameter for a

given centrality class. Strong correlation between different regions in plane ~R2

Can be eliminated using impact parameter dependent background subtraction. Poissonian fluctuations of uncorrelated particles

E = N [<pT>2 +pT2]

~R Correlated particles from common source (low-ET jets)

~R

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Jet finder in HI environment: Principle

Other algorithms have been tested successfully

FASTJET kT-algorithm (M. Cacciari, G. Salam)

Deterministic annealing (D. Perrino) Important because they show different

systematics for the background subtraction)

Loop1: Background estimation from cells outside jet conesLoop2: UA1 cone algorithm to find centroid

using cells after background subtraction

Rc

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CMS projected performance

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Jet position resolution

Jet energy resolution

Standard ATLAS solution -cone algorithm (R = 0.4) - is intensively

studied with different samples

Jet finding & energy measurement work for ET > 40 GeV (15 GeV in pp)

ATLAS projected performance

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New challenges for ALICE

Existing TPC+ITS+PID || < 0.9 Excellent momentum

resolution up to 100 GeV Tracking down to 100 MeV Excellent Particle ID

New: EMCAL Pb-scintillator Energy resolution ~15%/√E Energy from neutral particles Trigger capabilities

central Pb–Pb

pp

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Expected resolution including EMCAL

Jet reconstruction using charged particles measured by TPC + ITS And neutral energy from EMCAL.

Attention: ALICE quotes fluctuations relative to ideal jet with R = 1.0

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Measurement of the longitudinal jet structuredN

/d

2 GeV 1GeV

Ideal: No background Background estimated for Pb-Pb using HIJING

2 GeV 1GeV

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Measurement of the longitudinal jet structureStatistical error for Ejet = 100 GeV, 104 events

log(E/GeV)

log(

dN/d

E)

Background fluctuates up Background fluctuates down

Bias towards higher Bg

Systematics of Background Subtraction

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Measurement of the longitudinal jet structure

Robust signal but underestimation of jet energybiases to lower values.

-jet correlation

E = Ejet

Opposite direction Direct photons are not perturbed by the medium Parton in-medium-modification through the fragmentation functionCaveats

StatisticsSystematics from fragmentation photons

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Summary

We can look forward to very interesting physics with reconstructed jets in Heavy Ion collisions at the LHC High rates providing sufficient energy lever-arm to map out the

energy dependence of jet quenching. Large effects: Jet structure changes due to energy loss and the

additional radiated gluons. Experiments suited for jet measurements in Heavy Ion Collisions

ATLAS and CMS: larger acceptance, more statistics. ALICE: excellent PID and low-pT capabilities