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Hard probes capabilities of ALICE:Jets and Direct Photons

Andreas Morsch

CERN, Geneva

Hard Probes 2006, Asilomar, June 9-15, 2006

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Outline

Jet Physics Jets at LHC: New perspectives and challenges High-pT di-hadron correlations

Reconstructed Jets

-Jet Correlations Summary

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Jet physics at LHC

As for RHIC energies, RAA at LHC will only give lower limit on transport parameter.

Reason: Surface and trigger bias We can reduce the trigger and surface bias

by studying reconstructed jets and increase sensitivity to medium parameters.

Using reconstructed jets we can study directly

Modification of the leading hadron Additional hadrons from gluon radiation Transverse heating.

s = 5500 GeV

A. Dainese, C. Loizides, G. Paic

Hump-backed plateau from toy model

= ln(Ejet/phadron)

Nuclear mod. Factor RAA vs <q>^

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Jet physics at LHC: New perspectives

ET > Njets

50 GeV 2.0 107

100 GeV 1.1 106

150 GeV 1.6 105

200 GeV 4.0 104

At LHC rates are high at energies at which jets 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.

To make use of the high rate we need trigger !

Pb-Pb

1 month of running|| < 0.5

More than 1 jet > 20 GeV per central collision

#Jets ET>ETmin

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Jet physics at LHC: New challenges

The high production rates also represent a challenge More than one particle pT > 7 GeV per event 1.5 TeV background energy in cone of R = 2+2 < 1 ! Challenge for jet reconstruction algorithms !

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 Low S/B in this region is a challenge !

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New challenges: Apparatus

Also preparing ALICE for jet physics represents a challenge.

Existing: Tracking system Momentum resolution < 6% up to

pT = 100 GeV For jet structure analysis

Tracking down to 100 MeV Excellent Particle ID

New: For improved energy resolution and trigger: EMCAL Pb-scintillator Energy resolution ~15%/√E

central Pb–Pb

pp

pT/pT

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Di-hadron correlations:from RHIC to LHC

Di-hadron correlations will be studied at LHC in an energy region where full jet reconstruction is not possible (E < 30 GeV).

What will be different at LHC ? Number of hadrons/event large

Decreases S/B at LHC but increases also overall statistics The width of the away-side peak increases to higher order processes Wider -correlation (loss of acceptance for fixed -widow) due to smaller xB

Power-law behavior of x-section (d/dpT ~ 1/pTn) changes from n = 8 at RHIC

and n = 4 at LHC Changes the trigger bias on parton energy

See also, K. Filimonov, J.Phys.G31:S513-S520 (2005)

PYTHIA 6.2

Azimuthal correlation baseline

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Scaling from RHIC to LHC

S/B and significance for away-side correlations can be estimated by scaling rates between RHIC and LHC Ratio of inclusive hadron cross-section Replace N(pT) ~1/pT

8 by ~1/pT4

pTtrig > 8 GeV

RHIC/STAR-like central Au-Au (1.8 107 events)

LHC/ALICE central Pb-Pb (107 events), no-quenching

From STAR pTtrig = 8 GeV/c

1/2550

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Di-hadron correlations with ALICE

STAR LHC, ALICE acceptanceHIJING Simulation

“Peak Inversion”

O(1)/2

4 105 events

M. Ploskon, ALICE INT-2005-49

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Under study

For pT < 7 GeV many particles per event

Look for other possibilities to quantify jet-like correlations Example: Averaged Power-spectra (auto-correlations)

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The biased trigger bias

hep-ph/0606098

pTtrig > 8 GeV

<pTpart> is a function of pT

trig but also pTassoc, s, near-side/away-side, E

See also, K. Filimonov, J.Phys.G31:S513-S520,2005

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From di-hadron correlations to jets

Strong bias on fragmentation function … which we want to measure

Very low efficiency, example: 1.1 106 Jets produced in central Pb-Pb collisions (|| < 0.5) ~1500 Jets selected using leading particles pT > 60 GeV

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Reduction of the trigger biasby collecting more energy from jet fragmentation…

Unbiased parton energy fraction - production spectrum induced bias

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How to reconstruct jets in HI environment: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

Background: E ~ R2

1.5 TeV in cone of R = 1

85% of jet energy

Jets can be reconstructed using reduced cone size, but what is the energy resolution ?

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What determines the energy resolution ? There exist different kind of energy fluctuations that contribute to the intrinsic

energy resolution in HIC 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 Out-of-cone Fluctuations

pT > 0 GeV1 GeV2 GeV

Resolution limited by out-of-conefluctuations common to all experiments !

Ejet = 100 GeV

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Reconstructed energy for monochromatic jets

ET = 100 GeV

E/E ~ 50%

E/E ~ 30%

Tail towards higher energies = Trigger bias

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Expected resolution including EMCAL

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

Sarah Blyth, QM2004

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Trigger performance

Background rejection set to factor of 10=>HLT

Centrality dependent thresholds on patch energy

A. Mischke and P. Jacobs, ALICE INT-2005-50

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ALICE performance studiesWhat has been achieved so far ?

Full detector simulation and reconstruction of HIJING events with embedded Pythia Jets

Implementation of a core analysis frame work Reconstruction and analysis of charged jets.

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Energy spectrum from charged jets

Cone-Algorithm: R = 0.4, pT > 2 GeV

Selection efficiency ~30% as compared to 6% with leading particle !No deconvolution, but GaussE-n ~ E-n

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Jet structure observables

Background subtraction under study.

Bump from background

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Hump-back plateau

Bias due to incomplete reconstruction.

Erec > 100 GeV

Statistical error

2 GeV

104 events

High z (low ): Needs improved resolution (EMCAL). Low z (high ): Systematic error is a challenge, needs reliable tracking.

Also good statistics (trigger is needed)

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jT-Spectra

Statistical error

104 events

jT

Background small where transverse heatingis expected.

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More to come …

Dijet correlations “Sub-jet” Suppression ?

Look for “hot spots” at large distance to jet axis Small formation time

Can we observe ~10 GeV parton suppression within 100 GeV jets ?

R0 = 1fm

tform = 1/(kT)tsep = 1/

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Photon-tagged jets

Dominant processes:

g + q → γ + q (Compton)

q + q → γ + g (Annihilation)

pT > 10 GeV/c

-jet correlation E = Ejet

Opposite direction

Direct photons are not perturbed by the mediumNo surface bias

Parton in-medium-modification through the fragmentation function D(z), z = phadron/E

min max

IP

PHOS

EMCal

TPC

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• Pb-Pb collisionsR = 0.2, pT

thres = 2 GeV/c• Identification Probability 50 %• Misidentification 7 %• Signal/Background 4.2

• pp collisionsR = 0.2, pT

thres = 0.7 GeV/c• Identification Probability 100 %• Misidentification 4.5 %• Signal/Background 13

G. Conesa, ALICE-INT-2005-014, HCP 2005 G. Conesa, ALICE-INT-2005-014, HCP 2005 proceedingsproceedings

Promp photon identification:Promp photon identification:Isolation cut methodIsolation cut method

Prompt Prompt are likely to be produced isolated are likely to be produced isolated Two parameters define isolation:

Cone size R pT threshold candidate isolated if:

no particle in cone with pT > pTthres

pT sum in cone, pT < pTthres

PHOS

R

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Identifying prompt in ALICE

x5signal

Statistics for on months of running:2000 with E > 20 GeV

E reach increases to 40 GeV with EMCAL

Prompt reach ~ 100 GeV

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Fragmentation function

quenched jet

non-quenched

Pb-Pb collisions

Background

Signal

HIC background

Sensitivity ~ 5% for z < 0.4

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Summary

Copious production of jets in PbPb collisions at the LHC < 20 GeV many overlapping jets/event

Di-hadron correlations Background conditions require jet identification and reconstruction in

reduced cone R < 0.3-0.5 ALICE will measure jet structure observables (jT, fragmentation

function, jet-shape) for reconstructed jets. High-pT capabilities (calorimetry) needed to reconstruct parton energy Good low-pT capabilities are needed to measure particles from medium

induced radiation. In this sense ALICE is now optimized for jet studies in HIC

ALICE can measure photon tagged jets with E > 20 GeV (PHOS + TPC) E > 40 GeV (EMCAL+TPC)

Sensitivity to medium modifications ~5%