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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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  • 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 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
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
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
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
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.

rhic jet studies with leading particles


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.
sensitivity to transport coefficient

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
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
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
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


Direct measurement of

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

jet physics at lhc rates

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
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
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.
optimal cone size

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
  • 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
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

atlas 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)

new challenges for alice

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
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
Measurement of the longitudinal jet structure

2 GeV 1GeV

2 GeV 1GeV



Background estimated for

Pb-Pb using HIJING

Ideal: No background

measurement of the longitudinal jet structure1


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 structure2


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