Photon physics in ALICE

D.Peressounko

RRC “Kurchatov institute”

for the ALICE collaboration

Photons in heavy ion collision

Photon physics with ALICE

Direct photons:

photons not originated in hadrondecays.

- prompt photons:

photons created in collisions of

incoming partons and in parton

fragmentation

- thermal photons:

thermal emission of hot matter

Isolated photons:

photons without hadron activity insome cone

 prompt

isolated

thermal

Spectrum: temperature, fireball lifetime

RAA: control of initial state

Collective flow: flow development on earlystages

HBT: space-time dimensions of hot matter

Jet tagging: calibrated jets, fragmentationfunction modification in matter

ALICE setup for 2010

4/11 EMCAL

=40, ||<0.7

3/5 PHOS

=60, ||<0.12

Full TPC+ITS

=360, |TPC|<0.9, |ITS|<1.2

Photon physics with ALICE

Photon physics with ALICE

Calorimeters: PHOS and EMCAL

PHOS

EMCAL

E/E (%)

x (mm)

Rip (cm)

460

428

E/E (%)

x (mm)

Rip (cm)

$C:\Users\Dima\Desktop\CaloConv\ConvPlots\figsCC101007-HardProbesConference\Pi0_data_InvMassFullPtWithoutBG_90035620401003321.gif$

Photon physics with ALICE

Conversion method

$C:\Users\Dima\Desktop\CaloConv\ConvPlots\figsCC101007-HardProbesConference\XY_distribution_conference.gif$

$C:\Users\Dima\Desktop\CaloConv\ConvPlots\figsCC101007-HardProbesConference\Photon_RecoEff_conference.gif$

Photon physics with ALICE

Spectrum of direct photons: PHENIX

Tini = 300 to 600 MeV

 = 0.15 to 0.5 fm/c

D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006)

Spectrum of direct photons: ALICE

Photon physics with ALICE

[[1] N. Armesto, (ed.) et al. F.Arleo et al,

J.Phys.G35:054001, 2008

2+1 hydro, Tin=650 MeV, reach RHG EOS

pQCD: CTEQ6.5M + nDSg, AKK+c=50 GeV

[[2] F. Arleo et al.,(Yellow Report)hep-ph/0311131

pQCD:CTEQ5M,KKP(BFGII for ),

EKS98+Eloss(0 for )

[[3] S.Tubide et al., Phys.Rev.C72:014906,2005

To estimate expected systematic error extrapolate PHENIX results to ALICEaccounting S/Bg ratio, difference in amount of material, resolutions etc.

Photon physics with ALICE

Direct photon HBT

Most probable

yield

The lowest

yield

Correlationmethod:

Predictions

hadronicgas

QGP

sum

pQCD

M.M. Aggarwal etal.,Phys.Rev.Lett.93:022301,(2004)

86Kr+natNi

@ 60 AMeV

181Ta+197Au

@ 40 AMeV

M.Marques et al.,(TAPS collaboration)PRL 73 (1994) 34.

Photon physics with ALICE

Direct photon HBT in ALICE/PHOS, MCsimulations

Photon physics with ALICE

Direct photons and hadron RAA in AA collisions: RHIC

[PHENIX, QM2009]

Direct photons scale as with Npart and provide calibration of the initial state ofAA collisions.

Direct photon RAA in ALICE

Photon physics with ALICE

F. Arleo et al. Yellow report,

CERN-2004-009-D, hep-ph/0311131

~measurable level

So far ALICE collected ~15 nb-1

Isolated photons

Photon physics with ALICE

In pp collisions:

In PbPb collisions

Due to underlying event oneshould use fixed cut

Due to limited acceptance of ALICE use R = 0.4

hep-ex/0609031

Isolation of fragmentation photons

‐ The fragmentation channel dominatesal low pT

‐ After ~ 35 GeV the Compton channeldominates.

Photon physics with ALICE

Isolated

All direct photons

- Increase the Compton (and annihilation)contributions.

‐ Decrease strongly the fragmentation

channel

Raphaelle Ichou, Ph.D thesis 2010

Isolated photons: spectrum

Photon physics with ALICE

Raphaelle Ichou, Ph.D thesis 2010

Isolated direct photons

Photon physics with ALICE

Y. Mao et al.,

Eur.Phys.J.C57:613-619,2008

IC: R =0.3, (pT)=2 GeV/c

IC: R =0.2, pT>2 GeV/c

G. Conesa et al.,

ALICE-INT-2005-014, NIM A 580 (2007) 1446

2 PHOS modules

5 PHOS modules

Photon physics with ALICE

Isolated photons: jet tagging

xEz

pout

pTa

XE= -pTh · pT / |pT|2

pTt

• Direct access of jet modification & medium response

• Clean way to measure Fragmentation Functions:

approximate z with xE

- Caveat: kT smeares relation z<-> xE

• Systematic control on geometrical bias

Look at modification of FF in AA collisionswith respect to pp

Photon physics with ALICE

Gamma-jet correlations: PHENIX

A. Adare et al (PHENIX) PRC 80, 024908 (2009)

M.Connor @ Hard Probes 2010

Photon physics with ALICE

Correlation function with direct photons

• Started analysis

• First results are promising…

Y.Mao @ Hot Quarks 2010

Correction functions with isolated triggers

• Correlation of Charged Tracks / PHOS clusters / EMCAL clusters with charged hadronsmeasured in TPC+ITS.

•Left Plot: Near (mainly 0 for EM calorimeters) and away side correlation shows ajet-like structure

•Right plot: Applying isolation cuts to clusters or 0, the away side correlationremains: single pi0 - jets and/or direct-photon - jet ?

• Consolidate the isolation cut, extract kT, FF.

performance

01/09/2010

performance

01/09/2010

pp @ √s= 7 TeV

Photon physics with ALICE

Y.Mao @ Hot Quarks 2010

Photons provide the possibility to study alarge variety of characteristics of heavy-ioncollisions

ALICE measures photons in detectors basedon different technologies which providereliable cross-check

Ongoing analyses look promising.

Photon physics with ALICE

Conclusions

Backup slides

Photon physics with ALICE

-h correlation in pp and AA

Photon physics with ALICE

xEz

pout

pTa

XE= -pTh · pT / |pT|2

EPJC (2008) 57: Y. Mao

background photons

background soft hadrons

DAA = CFAA/CFpp

pTt

PYQUEN

-hadron correlations

Photon physics with ALICE

G. Conesa, Proceedings of Science, PoS (HIGH-pTLHC) 003

All associated charged hadrons with pT > 2 GeV/c

PYTHIA pp collisions √s=14 TeV

Isolated Spectra in EMCAL

pp @ √14 TeVpp @ √14 TeV

PbPb @ √5.5 TeVPbPb @ √5.5 TeV

PbPb @ √5.5 TeV, qhat = 50PbPb @ √5.5 TeV, qhat = 50

pp = PYTHIA

PbPb =PYTHIA (signal) +HIJING (UE)Full reconstruction in ALICE

-jet (signal); jet-jet (0 –hadron background)

Photon physics with ALICE

Influence of isolation on different sources

Pythia MSUB(14) =1 : Annihila5on ON

Pythia MSUB(29) =1 : Compton ON

PYTHIA6.420 :

* UE Perugia hard :

more ISR/FSR, less MPI & beam remnants

* UE Perugia sol :

less ISR/FSR, more MPI & beam remnants

• 80%‐100% of Compton and

annihila5on photons

• 60% of fragmenta5on

photons

• 70%‐85% of total photons

• 20%‐1% of π0

Photon physics with ALICE

Raphaelle Ichou, Ph.D thesis 2010

PHOton Spectrometer: PHOS

High granularity and resolutionspectrometer:

10,752 (17,920) lead-tungstate crystals(PbWO4), 3(5) modules (5664 crystals permodule)

crystal size: 22  22  180 mm3

depth in radiation length: 20

Distance to IP: 4.4 m

Acceptance:

pseudo-rapidity [-0.12,0.12]

azimuthal angle 60º(100o)

For E > 10 GeV,

E/E < 1.5% and x = [0.5,2.5] mm

Focus on low and moderate pT

High resolution 0 and 

Thermal photons

26/25

CPV (not installed yet)

Crystals

EMC

Photon physics with ALICE

27/25

Module:

2x2 towers

•Coverage =1.4, =40o (107o)

•Granularity  = = 0.014

•Resolution E/E=11%/√E,

x=[3,50]

•Focus on moderate to high pT

0 and 

prompt direct photon

jet

Pb/Sc Shashlik

–13K channels

–4 SM for 2009-2010 runs

–Complete for 2011? runs

EMCAL

Photon physics with ALICE