Jets in heavy ion collisions at the lhc
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Jets in Heavy Ion Collisions at the LHC. Andreas Morsch CERN. 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 ?

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Jets in Heavy Ion Collisions at the LHC

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Jets in Heavy Ion Collisions at the LHC

Andreas Morsch



  • 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 ?

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-DE)+ soft gluons (DE)

Medium induced parton energy loss

Example: BDMPS

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

Coherent sum over scatterings with

free path length l and mean qT transfer m

Medium characterized by

transport coefficient:

Expect large effects !

Needs large range of E to measure DE(E)

Consequences for the jet structure

Decrease of leading particle pT

Increased mult. of low-pT

Particles from radiation.

Increase of pT rel. to jet-axis

Energy outside jet cone

Dijet energy imbalance

and acoplanarity



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

Jets at RHIC

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

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.


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

Pedestal&flow subtracted

RHIC: Jet studies with leading particles

Suppression of inclusive hadron yield

Disappearance of away-side correlations

  • 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.

Eskola et al., hep-ph/0406319

RAA~0.2-0.3 for broad range of

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.

Surface emission bias limits

sensitivity to

Bias from the production spectrum

100 GeV Jet

Mean value shifts to

pLeading/Eparton =0.6

  • Strong bias on fragmentation function

    • … which we want to measure

  • But also low efficiency since only tail is relevant.

pLeading [GeV]

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

Jet structure observables

Longitudinal Structure

Transverse Structure

Borghini,Wiedemann, hep-ph/0506218

Salgado, Wiedemann, Phys. Rev. Lett. 93: 042301 (2004)

Sensitive to out-of-cone radiation.

I. Lokhtin


Direct measurement of

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

A. Accardi et al., hep-ph/0310274

CERN TH Yellow Report

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.

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 = Dh2+Df2 < 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.


Quenched (AliPythia)

Quenched (Pyquen)

pT < 2 GeV

* For dN/dy = 5000.

Jet reconstruction in Heavy Ion Collisions

  • How to reconstructs jets above a large fluctuation background (DEBg) ?

    • Restrict identification and reconstruction to domain in which Emeas >> DEBg

    • 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.

Jets reconstructed from charged particles:

Energy contained in sub-cone R

Need reduced cone sizes and transverse momentum cut !

Optimal cone size

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.

E ~ R2

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

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 h-f plane

    • ~R2

    • Can be eliminated using impact parameter dependent background subtraction.

  • Poissonian fluctuations of uncorrelated particles

    • DE = N[<pT>2 +DpT2]

    • ~R

  • Correlated particles from common source (low-ET jets)

    • ~R

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 cones

Loop2: UA1 cone algorithm to find centroid

using cells after background subtraction

CMS projected performance


ATLAS projected performance

Standard ATLAS solution -cone algorithm (R = 0.4) - is intensively studied with different samples

Jet position resolution

Jet energy resolution

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

central Pb–Pb


New challenges for ALICE

  • Existing TPC+ITS+PID

    • |h| < 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

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

Measurement of the longitudinal jet structure

2 GeV 1GeV

2 GeV 1GeV



Background estimated for

Pb-Pb using HIJING

Ideal: No background


Background fluctuates down

Background fluctuates up

Bias towards higher Bg


Measurement of the longitudinal jet structure

Statistical error for Ejet = 100 GeV, 104 events

Systematics of Background Subtraction


Measurement of the longitudinal jet structure

g-jet correlation

  • Eg = Ejet

  • Opposite direction

  • Direct photons are not perturbed by the medium

  • Parton in-medium-modification through the fragmentation function

  • Caveats

    • Statistics

    • Systematics from fragmentation photons

  • Robust signal but underestimation of jet energy

    biases x to lower values.


    • 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

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