Prompt photon physics in ALICE: g -jet & g -hadron correlations

11/45/4526/03/200726/03/2007 Gustavo Conesa Balbastre @ High pT physics at LHC workshopGustavo Conesa Balbastre @ High pT physics at LHC workshop

Prompt photon physics in Prompt photon physics in ALICE: ALICE:

-jet & -jet & -hadron correlations-hadron correlations

A feasibility and performance study

Gustavo Conesa Balbastre


OutlookOutlook

Motivation: Photons in heavy-ion collisions Motivation: Photons in heavy-ion collisions Photons sourcesPhotons sources RHIC measurementsRHIC measurements

ALICE experiment: CalorimetersALICE experiment: Calorimeters Prompt photon identification Prompt photon identification Prompt photon correlations Prompt photon correlations ConclusionsConclusions


Why Why ??

Study the properties of matter Study the properties of matter at high density and temperature with: at high density and temperature with: JetsJets

Partons (jets) suffer energy loss traversing Partons (jets) suffer energy loss traversing the medium the medium Jet multiplicity and Jet multiplicity and energy redistribution energy redistribution Jet-QuenchingJet-Quenching

PhotonsPhotons Production unperturbed by the mediumProduction unperturbed by the medium

Prompt photons: Test QCD, Prompt photons: Test QCD, -jet events -jet events Jet-QuenchingJet-Quenching Production modified or created in the mediumProduction modified or created in the medium

Fragmentation photonsFragmentation photons Quenching Quenching Bremstrahlung, Jet-conversion Bremstrahlung, Jet-conversion EnhancementEnhancement

Decay photons (neutral mesons) Decay photons (neutral mesons) Observation of hadron Observation of hadron suppression suppression Jet-QuenchingJet-Quenching

Fragmentation Jet

Prompt

0


Direct prompt OS)]: g+q +q (Compton) q+q +g (Annihilation)

Parton in-medium-modification imprinted in the final hadronic state (jet-quenching).

Prompt photons are not perturbed by the medium.

Direct thermal photons Equilibrium: QGP and hadron gas.

Thermal emission from the medium.

Photon Photon ssourcesources

Pre-equilibrium

Equilibrium

Freeze-out

0

Prompt

AA BBColliding ions


Fragmentation prompt OS)]:

Bremsstrahlung production modified by the medium

Jet re-interaction q+gmedium +q

q+qmedium +g

Photon Photon ssourcesources

Prompt

AA BB

Fragmentation

Pre-equilibrium

Equilibrium

Freeze-out

Colliding ions

0


Photon sourcesPhoton sources

Photons carry unperturbed information

on the hot and dense medium (direct photon),

and reveal medium induced modifications (decay photons).

Photon sources in the initial stage of the collision when the system is hottest (pQCD prompt, jet re-interaction, QGP thermal).

Hadron gas + decay: later phase of the collision Background

E

Rat

e

Hadron Gas Thermal Tf

QGP Thermal Ti

Jet Re-interaction √(Ti x √s)

Bremsstrahlung (jet -quenching)

pQCD LO (Compton) + NLO (fragmentation)


Jet

Jet

A B

0

• Decay Photons :• Neutral mesons, 0 and , decay mainly into

2 .• Main photon source in heavy-ion collisions,

background for direct .• Mesons production suppressed (RHIC) by

medium effects (jet-quenching).• Only identified hadronic probe measurable

up to very high pT

Photon sourcesPhoton sources

Pre-equilibrium

Equilibrium

Freeze-out

Colliding ions


Decay Decay vs Direct vs Direct

p+p collisions:p+p collisions: mainly mainly 00

A+A collisions:A+A collisions: Jet-QuenchingJet-Quenching RHIC:RHIC:

NN > N > N for p for pT T > 10 GeV/> 10 GeV/cc LHC:LHC:

NN > N > N for p for pT T > 100 GeV/> 100 GeV/cc

PID: Shower shape + PID: Shower shape + Isolation cutIsolation cut..

0 = 0,01-0,1

Photon Yellow Report Photon Yellow Report hep-ph/0311131hep-ph/0311131


Jet-quenching at RHICJet-quenching at RHIC s = 200s = 200AA GeV GeV

RAA reference: pQCD calculation

Yesterday

RAA reference: fit to p+p measurements

Today

Jet

Jet

Hadrons suppression factor 5!No suppression of , as expected?

CERN Heavy Ion Forum, March 6, 2007 -- G. David, BNL


CERN Heavy Ion Forum, March 6, 2007 -- G. David, BNL

RAA with pQCD

RAA with p+p data

S. Turbide, Phys. Rev. C72 (2005) 014906

F. Arleo, JHEP09 (2006) O15

W. Vogelsang, NLO pQCD + isospin

Direct Direct R RAAAA in Au+Au – Theory and Experiment in Au+Au – Theory and Experiment


OutlookOutlook

Motivation: Photons in heavy-ion Motivation: Photons in heavy-ion collisions collisions

ALICE experiment: CalorimetersALICE experiment: Calorimeters Prompt photon identification Prompt photon identification Prompt photon correlationsPrompt photon correlations ConclusionsConclusions


Solenoid magnet 0.5 T

MUON Spectrometer

PHOS

HMPID

ALICE: ALICE: AA LLarge arge IIon on CCollider ollider EExperimentxperiment

• ITS • TPC• TRD• TOF Central tracking system


E > 10 GeV E/E < 1.5%, x =[0.5,2.5] mm

Detectors to be used Detectors to be used

=120º || < 0.7

=100º || < 0.12

=360º || < 0.9

PHOS

TPC

EMCal

p/p = 2%,

=1.1º


PPHOHOton Spectrometer: PHOSton Spectrometer: PHOS

High granularity detectorHigh granularity detector:: 1717,,920920 lead-tungstate crystals lead-tungstate crystals

(P(PbbWOWO44), 5 modules (), 5 modules (56566464))

crystal sizecrystal size:: 22 22 22 22 180 mm 180 mm33

depth in radiation lengthdepth in radiation length:: 2020

DDistance to IP:istance to IP: 4.4 m4.4 m

AcceptanceAcceptance:: pseudo-rapiditypseudo-rapidity [-0.12,0.12] [-0.12,0.12] aazimuthal anglezimuthal angle 100100oo

CCharged harged PParticle article VVetoeto,, CPVCPV multi-wire particle gas chambermulti-wire particle gas chamber

High Resolution spectrometer

CPV

CrystalsEMC

ALICE PPR chapter 5


ElectroMagnetic Calorimeter: EMCalElectroMagnetic Calorimeter: EMCal

Description:Description:

12,67212,672 towers (scintilator-Pb), towers (scintilator-Pb),

10 modules (10 modules (24 24 48 48) + ) +

2 half size modules (2 half size modules (12 12 48 48) )

tower sizetower size:: 60 60 60 60 250 mm 250 mm33

depth in radiation lengthdepth in radiation length:: 2222

Distance to IP:Distance to IP: 4.28 m4.28 m

Acceptance:Acceptance:

pseudo-rapiditypseudo-rapidity [-0.7,0.7] [-0.7,0.7]

azimuthal angleazimuthal angle 110110oo

EMCal is 7 times larger than PHOSEMCal is 7 times larger than PHOS but it is a but it is a

moderate energy resolution calorimetermoderate energy resolution calorimeter

http://rhic23.physics.wayne.edu/twiki/pub/Alice/ReviewDocs/MIE_Proposal_12-5-05.pdf


PHOS bPHOS beam-test and eam-test and ssimulationsimulations

Energy resolution Position resolution

E > 10 GeV E/E < 1.5% E > 10 GeV x =[0.5,2.5] mm

Heavy-ion environment worsens the resolution by less than 2%

ALICE PPR chapters 5


PHOS bPHOS beam-test and eam-test and ssimulationsimulations

Fixed target experiment-+12C0()+X, E- = 6 GeV

Inv. Mass Resolution is 3-5% in 0.5 < E < 30 GeV in PHOS simulations

ALICE PPR chapters 5


EMCal beam-testEMCal beam-test

E > 10 GeV E/E < 3% E > 10 GeV x < 3.5 mm


OutlookOutlook

Motivation: Photons in heavy-ion collisions Motivation: Photons in heavy-ion collisions ALICE experiment: CalorimetersALICE experiment: Calorimeters Prompt photon identificationPrompt photon identification

Isolation cut methodIsolation cut method Prompt photon correlationsPrompt photon correlations ConclusionsConclusions


Photon Photon identificationidentification

We can discriminate e and from anything else : “PCA” or “Bayesian”, based on:

CPVCPV : Charged particle rejection : Charged particle rejection

TOFTOF : Rejection of massive low : Rejection of massive low ppTT particles particlesEMCEMC : Hadron rejection via : Hadron rejection via shower topologyshower topology

Algorithms tuned with simulations:Few % hadron contamination in HI environment.Designed to distinguish high-energy and 0.

0 decay overlaps in PHOS from 30 GeV (15 GeV in EMCal)High identification efficiency, ~ 60%, and misidentification smaller than 10 % for 30 GeV < E < 100 GeV.

Not enough for Prompt identification: We need Isolation Cut Method

ALICE PPR chapters 5 and 6, photon sections


Prompt Prompt identification: identification:Event generationEvent generation

p-p collisionsp-p collisions: : PYTHIA 6.2PYTHIA 6.2 as event generator, as event generator, p-p collisions p-p collisions @ @ 5.5 TeV:5.5 TeV:

+jet in final state +jet in final state – jet – jet.. Prompt is the is the signalsignal under study: under study: 20 GeV < E 20 GeV < E < 100 GeV. < 100 GeV.

2 jets in final state 2 jets in final state jet –jet – jetjet.. These events constitute the These events constitute the backgroundbackground::

high-pT 0 OS)] and and bremsstrahlung OS)]: :

30 GeV < E 30 GeV < E jetjet < 300 GeV. < 300 GeV.

Pb-Pb collisionsPb-Pb collisions:: p-p collisions p-p collisions ++ underlying event for underlying event for Pb-Pb collisions @ 5.5 TeVPb-Pb collisions @ 5.5 TeV,, HIJINGHIJING, dN/dy~6000., dN/dy~6000. Binary scaling from p-p collisionsBinary scaling from p-p collisions,, minimum biasminimum bias.. No medium effects !No medium effects !

ALICE-INT-2005-014ALICE-INT-2005-014


Prompt Prompt identification: identification:Generated siGenerated signalgnal and b and backgroundackground

Background

Factor 5 suppression

• Direct results from

PYTHIA, no detector

response function,

corrected for detector

acceptance.

• Background =

decay + bremsstrahlung

• PHOS identifies

efficiently through

Shower Topology: Not

enough to tag them as

prompt.

Signal



Overlapped Clusters RejectionOverlapped Clusters Rejection

Signal/Background: • as• as

• Too much 0 background !• New ID criteria to be found

Pb-Pb collisions

1-d shower shape analysis



•pT threshold candidate isolated if:

• no particle in cone with pT > pT thres

• pT sum in cone, pT < pTthres

Our signal

Bremsstrahlung (Background!)• Two parameters define isolation:

•Cone size 22R

R

Prompt Prompt are likely to be produced isolated are likely to be produced isolated..

Prompt Prompt identification: identification:Isolation cut Isolation cut methodmethod

PHOS

TPC

candidate

IP

• Pb-Pb collisionsR = 0.2, pT

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

• pp collisionsR = 0.2, T

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



Final identified Final identified prompt prompt spectrumspectrum Annual statisticsAnnual statistics

Particles identified as (medium purity)IC: R =0.2, pT>2 GeV/c

Factor 5 suppression

SignalBackground

Statistics limits to ~100 GeV

Corrected spectrum, systematic errors



Quality of dataQuality of data

Systematic error suppressed a factor 5 by quenching



OutlookOutlook

Motivation: Photons in heavy-ion collisions Motivation: Photons in heavy-ion collisions ALICE experiment: PHOSALICE experiment: PHOS Prompt photon identification Prompt photon identification Prompt photon correlationsPrompt photon correlations

-jet-jet-hadron -hadron

ConclusionsConclusions


Hadron redistribution can be best measured in the Hadron redistribution can be best measured in the Fragmentation Function... Fragmentation Function... If we know EIf we know Epartonparton..

HI environment limits the precision on the energy of the reconstructed jet/parton:

Measure Eprompt Eparton

Fragmentation Jet

Prompt

Why Why -hadron/jet correlations?-hadron/jet correlations?

Study medium modification in fragmentation function Study medium modification in fragmentation function (R(RAAAA of FF) from isolated of FF) from isolated -jet and isolated -jet and isolated -hadron -hadron

correlations.correlations.

0

Medium effects redistribute (qL) the parton energy, Eparton, inside the hadron

jet (multiplicity, kT).

^̂


Tagging jet with photonTagging jet with photon

Search identified prompt photon (PHOS) with largest pT (E > 20 GeV).

Strategy Strategy (event by event)(event by event)::min max

leading

Search leading particle : -leading180º Eleading > 0.1 E

R

Reconstruct the jet: Particles around the leading with pT > 0.5

GeV/c, inside a cone of R = 0.3. 2 configurations: charged and neutral

hadrons (TPC+EMCAL) and charged only (TPC).

IP

PHOS

EMCal

TPC



Pb-Pb collisions, pT, part > 0.5 GeV/c

Reconstructed jet selectionReconstructed jet selection40 GeV jets40 GeV jets

TPC alone

TPC+EMCAL

p-p collisions, pT, part > 0.5 GeV/c

TPC alone

TPC+EMCAL

Pb-Pb collisions, pT, part > 2 GeV/c



Fragmentation functionFragmentation function

• E > 20 GeV/c; TPC+EMCal detect jet particles, PHOS

Pb-Pb collisions

Background

Signal

HIC background

Any neutral signal in PHOS

z = pT, jet particle /E

Prompt identified in PHOS

If signal is quenchedIf signal is quenched



Charged + EM

-tagged FF R-tagged FF RAAAA

With quenched 0

PNNCAB

PAB

AB dT

dNR

Systematic errors due

to jet(0)-jet background

Sensitive to medium modifications at low z if larger than ~5% in both configurations.



-hadron correlations-hadron correlations

We could do the same study in a simpler way: We could do the same study in a simpler way: tagging hadrons opposite to the isolated tagging hadrons opposite to the isolated ..

Suggested by F. Arleo Suggested by F. Arleo et al.et al. in : in : JHEP 0609:015,2006, hep-ph/0601075 JHEP 0609:015,2006, hep-ph/0601075 JHEP 0411:009,2004, hep-ph/0410088 JHEP 0411:009,2004, hep-ph/0410088 hep-ph/0701207 hep-ph/0701207


F.Arleo et al. hep-ph/0701207

= Eparton



Requirements for good measurement:Requirements for good measurement: Perturbative direct photons.Perturbative direct photons.

At LHC At LHC ppTT > 20-30 GeV/c > 20-30 GeV/c

Perturbative hadron, no medium residues.Perturbative hadron, no medium residues. At LHC At LHC ppTT > 10 GeV/c > 10 GeV/c

Wide z range, ideally 0 Wide z range, ideally 0 z z Thus ideally : Thus ideally : minimumminimum ppTT>> minimum p>> minimum pTT

Reasonable counting ratesReasonable counting rates At LHC At LHC ppTT < 100 GeV/c < 100 GeV/c

Study made for pStudy made for pTT>20 GeV/c and p>20 GeV/c and pTT>70 GeV/c.>70 GeV/c.




-hadron correlation-hadron correlationppTT > 70 GeV/c – p > 70 GeV/c – pTT>10 GeV/c>10 GeV/c

Most of the z interval ... but limited counting rate.


Theoretical FF


-hadron correlation-hadron correlationppTT > 20 GeV/c – p > 20 GeV/c – pTT>10 GeV/c>10 GeV/c


Theoretical FF

No match with real FF ... but good counting rate.


-hadron correlation-hadron correlationfragmentation fragmentation contribution contribution

Much larger fragmentation component with pT>20 GeV/c.Decay photons will contribute much more.Isolation of direct photons will reject both.


-hadron correlation-hadron correlationQuenchingQuenching

Don’t see expected suppression in all region.

Weaker energy loss, but see larger z region.


The study claims that better concentrate in pT>70 GeV/c direct , and pT >10 GeV/c,but our calorimeters acceptance reduce counting rate:

PHOS 20< pT< 40 GeV/c

EMCal 20< pT< 60 GeV/c

We have to investigate results at lower pT cuts.

Basic yields for 5.5A TeV Pb+Pb collisionsBasic yields for 5.5A TeV Pb+Pb collisions

1k/year


-hadron correlation in ALICE -hadron correlation in ALICE

Search identified prompt photon (PHOS or EMCal) with largest pT (E> 20 GeV).

Strategy following François Arleo studies (event by event):

hadron

Search for all charged hadrons (TPC+ITS) or neutral 0 (EMCal or PHOS):

90º< -hadron < 280º pT hadron > 10 GeV/c

IP

PHOS/EMCal

EMCal/PHOS

TPC+ITS


OutlookOutlook

Motivation: Photons in heavy-ion collisions Motivation: Photons in heavy-ion collisions ALICE experiment: CalorimetersALICE experiment: Calorimeters Prompt photon identification Prompt photon identification Prompt photon correlations Prompt photon correlations ConclusionsConclusions


Conclusions Conclusions 11//33

Identification of 2Identification of 22 pQCD processes with 2 pQCD processes with Prompt photon identification: Isolation cut method.Prompt photon identification: Isolation cut method.

Efficiently rejects background.Efficiently rejects background. 20% of systematic error from left over background.20% of systematic error from left over background. Assuming quenching, systematic errors dramatically reduced in Assuming quenching, systematic errors dramatically reduced in

Pb-Pb collisions.Pb-Pb collisions. Statistics (PHOS acceptance) limits the measurement to energies Statistics (PHOS acceptance) limits the measurement to energies

below 100 GeV .below 100 GeV .

Photon-tagged algorithm to measure jet properties.Photon-tagged algorithm to measure jet properties. To measure the redistribution of fragmentation hadrons inside the To measure the redistribution of fragmentation hadrons inside the

jet (jet multiplicity, jet heating).jet (jet multiplicity, jet heating). EMCal helps to improve the background rejection.EMCal helps to improve the background rejection. RRFFFF shows a sensitivity of medium induced modification at the shows a sensitivity of medium induced modification at the

level of 5%.level of 5%.


Conclusions Conclusions 22//33

Within its present configuration and the developed Within its present configuration and the developed methodsmethods,, ALICE can measure photon (PHOS) ALICE can measure photon (PHOS) tagged jets (TPC) with energy ~20 GeV.tagged jets (TPC) with energy ~20 GeV.

Adding EMCal, due to the increased acceptance, Adding EMCal, due to the increased acceptance, measurements measurements of of photon (EMCal) tagged jets photon (EMCal) tagged jets (TPC) extended to ~ 40 GeV.(TPC) extended to ~ 40 GeV. Study of the jet-quenching over a broader energy Study of the jet-quenching over a broader energy

range.range.

In both cases the sensitivity to medium effects is In both cases the sensitivity to medium effects is of about 5 %.of about 5 %.


Conclusions 3/3Conclusions 3/3 -hadron correlation provides new insight in the -hadron correlation provides new insight in the

study of medium modificationsstudy of medium modifications

Optimum conditions would be at Optimum conditions would be at ppTT > 70 GeV/c > 70 GeV/c and and ppTT>10 GeV/c>10 GeV/c, but small statistics, , but small statistics, ppTT > 40- > 40-50 GeV/c50 GeV/c and and smaller psmaller pTT cutcut might be OK, to be might be OK, to be investigated. investigated.

Event production with the GRID started. Study Event production with the GRID started. Study -jet/hadron correlations with new more realistic -jet/hadron correlations with new more realistic simulations.simulations.

Prompt photon physics in ALICE: g -jet & g -hadron correlations

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