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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 ? - PowerPoint PPT Presentation
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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.
65
)/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