1Guénolé BOURDAUD

Guenole.bourdaud@subatech.in2p3.fr

Gamma-jet physics with the Electromagnetic Calorimeter

(EMCal) in ALICE experiment at LHC

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Hard Probes in QGP Study, historical view

~98%~ 50%~ 2%hard/totAlice was designed

before RHIC results.

Lessons from RHIC :

Need dedicated detectors for high

pT and Hard probes

> EMCal

QGPInitial state

(partonic) observations.

Explosion of hard probes

New matter state.Final state (hadronic)

observations.Emergence of hard probes.

Measurement

200820001994Global Observables

~1994~1990~1980Start of construction

LHCRHICSPS

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Hard probes : interest of LHC

Heavy ion dedicated experiment : ALICEMore jets, higher pT and access to gamma-jet correlations with enough statistics.

•2008 : first pp collisions at 14 TeV…•2009 : collisions Pb-Pb low luminositiy•2010 : collisions Pb-Pb high luminositiy

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Jet quenching at RHICHard scatterings in nucleon collisions produce jets of particles.

In the presence of a color-deconfined medium, the partons strongly interact loosing a significant part of their energy.

Energy loss : “Jet Quenching”

Redistribution of jet energy :- less particles at high pT : interaction with medium : seen @ RHIC+ more particles at low pT : radiations of gluons : visible @ ALICE ?

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ALICE Central tracking + EMCal :Well designed detectors to study jet quenching effect Alice central traking :

= +/- 0.9 = 360°Charged particlesHigh particle identification capabilitiesLow pT possibilitiesFor neutral particles :

PHOS : high granularity spectrometerEMCAL : large aceptance calorimeter

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ALICE Central tracking + EMCal :Well designed detectors to study jet quenching effect

EMCal & TPC :Possibility to reconstruct jet event by event.Full jet reconstruction (neutral and charged)Gamma detection (for gamma-jet).

EMCal :U.S.-Italy-France project = +/- 0.7 = 110°11 modulesReady for run 2009 (1 module)

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• Direct prompt • g+q +q (Compton) • q+q +g (Annihilation)

– Parton in-medium modification perturb final hadronic state (jet-quenching)

– Prompt photons are not perturbed by the medium– Prompt photons gives jet energy.

Initial Photon (Initial Photon ( prompt).prompt).

Initial Initial parton parton

Leading ParticleLeading Particle

JetJet

•Want to study in medium interaction by jet modification (p-p vs Pb-Pb)•Need to obtain the jet energy : gamma-jet is a good solution.

Gamma-jet

_

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gamma-jet in ALICE

Gamma-jet in EMCal is one order of magnitude higher than in PHOS.~ 10k events for energy higher than 30 GeV / year

Need the high geometrical acceptance of EMCalNot seen at RHIC (not enough statistic)

PHOS has a lower geometrical acceptance, can not see (enough) Gamma-jetsHard to observe : background from jet-jet and heavy-ion collision.

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)(

)(

JetT

particleT

ppx

x

Dashed red : quenched black : unquenched

Schematic example

Fragmentation function & Hump-backed Plateau

Depletion of High energy particles to increase the number of low energy particles

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180°

EMCalpT leading

> 10% pT prompt

pT Jet ~ pT prompt

Beam axis

TPC

Moyen

sAlgorithm for gamma-jet

reconstruction

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Gamma selection•Particle identification : Pid (based on Shower Shape Analysis : SSA)Method developped for EMCal, can distiguish , 0 and other hadrons.

Efficiency ~55 % and Purity ~70 % for Gamma energy higher than 30 GeV to avoid backgroud photons (thermal, decay, pi0 …)•Isolation cut : energy higher than 10% of the energy in a cone centered around it. If exact, is isolated and so considered as a prompt .

SSA

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Jet reconstruction

•Cuts & parameters•Azimutal correlation, angular selection : ( - jet) < 0,1 rad•Leading particle Elead/E < 0,1 • Reduced cone algorithme R=0.3 = (²+²) (backgroud study in progress)

•Try to use same parameters in p-p and Pb-Pb

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x

=ln(1/x)

1/N

dN

/d

Fragmentation function

Ejet=30GeV .

Humpbacked plateau

1/N

dN

/dx

Reconstruction of Fragmentation Fonction & humpbacked plateau.

)(

)(

JetT

particleT

ppx

quenched but no backgroundunquenched

Pythia simulation, full geant reconstructionQuenching with pyquen.I.P. Lokhtin, A.M. Snigirev, Eur. Phys. J. C 46

(2006)211-217 q ~ 30 GeV2/fmNo background71% of gammas found72 % of jet found if gamma found.

^

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Estimating the heavy-ion background

The modification of the hump-backed plateau is dominated by background in Pb-Pb collisions.Measured S/B :S/B ~ 10-2 for > 2.5 (pT< 2.46 GeV)

quenched signal only : no backgroundSignal + background

Measured S/B

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With the heavy-ion background & errorsThe modification of the

hump-backed plateau is dominated by background in Pb-Pb collisions.Need background corrections. Actually :Can see depletion of high energy particles (seen at RHIC)Need bkg. subtraction

unquenchedquenched with backgroundquenched but no background

ratio unquenched/quenchedWith backgroundNeed bkg substraction

ratio unquenched/quenchedWithout backgroundOnly Signal

=ln(1/x)

=ln(1/x)

1/N

dN

/d

1/N

dN

/dx

x

Hb.

pLa

t. PB

/p

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ConclusionWith the EMCAL, ALICE will greatly improve its direct photon and jet physics capabilities.•These new, more accessible, probes, will help ALICE to study the Quark Gluon Plasma in greater detail.• gamma-jets give access to redistribution of the jet energy. Show the depletion for high energy particles.•The low pT particles increase has to be studied by background subtraction.

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More …

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Jet-quenching at RHICs = 200A GeV

1/NtriggerdN/d()

Near side

Away side

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Jet-quenching at RHICs = 200A GeV

1/NtriggerdN/d()

Near side

Away side

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Jet-quenching at RHICs = 200A GeV

1/NtriggerdN/d()

The back to back jet correlation is lost due to hard interaction in plasma.

The rare process (Hard Scattering or “Jets”) is the probe of whether the soft production products form a medium.

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Conclusion – gamma-JETS

•It is possible to show the decrease of high energy particles, not yet the increase of low energy particle (Background subtraction in progress)•Need:

•Systematic study of cuts in algorithm•Jet-jet pollution of signal (isolation cut study)

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PID, shower shapeShower shape 0 :

Cluster in EMCal higher energy

°

°

°

Gustavo Conesa, thesis : University of Nantes and University of Valencia, 2005 ALICE-INT-2005-053

A tower

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Apport de EMCal dans ALICE :Apport de EMCal dans ALICE : Énergie totale du jet (E’+E) mesurée avec des biais réduits et une meilleure résolution Meilleure définition de la fonction de fragmentation, à bas z (dynamique de la perte d’énergie) Déclenchement efficace sur les jets d’énergie élevée Trajectographie centrale permet l’étude :

de la réponse du milieu (partie soft) de la composition des jets atténués

L’expérience idéale pour la physique des grands pL’expérience idéale pour la physique des grands pTT : : Calorimètres tels ceux d’ATLAS et de CMS

Résolution (détecteur, mesure des jets) meilleure Acceptance plus grande

Trajectographie et Identification de Particules d’ALICE

La réalité : des expériences complémentaires La réalité : des expériences complémentaires Atouts de ATLAS/CMS :

Taux élevés domaine cinématique étendu (> 350 GeV) Études -jet, Z-jet (avec des statistiques limitées)

Atouts de ALICE : cf ci-dessus

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TPC acceptance = 90%

ALICEdN/d=6000

CMSdN/d = 3200

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