Direct photon detection in pp and PbPb collisions in the ALICE experiment at LHC

Direct photon detection in pp Direct photon detection in pp and PbPb collisions in the ALICE and PbPb collisions in the ALICE

experiment at LHCexperiment at LHC

Yuri KharlovFor the ALICE PHOS collaboration

55thth International Conference on Perspectives in Hadronic Physics International Conference on Perspectives in Hadronic Physics

22-26 May 200622-26 May 2006

22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 22

OutlineOutline ALICE photon detectorsALICE photon detectors Physics motivationPhysics motivation Photon sourcesPhoton sources Expected ratesExpected rates Experimental methodsExperimental methods


ALICE experiment at LHCALICE experiment at LHC

ALICE

ALICE is a dedicated heavy-ion experiment at LHC.

Our aim is to study the physics of strongly interacting matter at extreme energy densities.

1000 scientists from 30 countries.


ALICE setupALICE setup

On ALICE physics see a talk by M.Monteno on 24 May


Photon detectors in ALICEPhoton detectors in ALICE

PHOS

EMCAL

See a talk by N.Bianchi on 23 May


PHOS crystalsPHOS crystals

Base element: PbWO4 crystal grown at “North Crystals” enterprise in Russia

RM=2.0 mm

X0=8.9 mm

=8.28 g/cm3

n=2.16size: 222216 cm3

PHOS has 5 modules installed at 4.6 m apart from the ALICE interaction point.

Each module contains 3584 crystals

Total number of crystals: 17920

PHOS aperture: ||<0.13, =100

Energy range: E<100 GeV


First PHOS module is assembled in First PHOS module is assembled in April-May 2006April-May 2006


Main physics interestsMain physics interests Photon are highly penetrating particles which escape Photon are highly penetrating particles which escape

from the hot nuclear matter almost intact and, from the hot nuclear matter almost intact and, therefore, carry undistorted information from any stage therefore, carry undistorted information from any stage of the nuclear matter evolutionof the nuclear matter evolution

Direct photons are produced in elementary interactions Direct photons are produced in elementary interactions acts in the nuclear matter:acts in the nuclear matter: in hard QCD processes, observed mainly at high pin hard QCD processes, observed mainly at high pTT; ; In emission of thermalized nuclear matter, observed mainly at In emission of thermalized nuclear matter, observed mainly at

low plow pTT serve as a probe for thermal properties of the early phase of the serve as a probe for thermal properties of the early phase of the nuclear reaction. Aka “nuclear reaction. Aka “thermal photonsthermal photons”.”.

Fragmentation photonsFragmentation photons Decay photons reveal medium-induced modifications of Decay photons reveal medium-induced modifications of

hadron properties.hadron properties. Interferometry of photons can be used as a tool to Interferometry of photons can be used as a tool to

measure geometrical size of the source.measure geometrical size of the source.See a talk by F.Arleo on 24 MaySee a talk by F.Arleo on 24 May


Photon sources (1)Photon sources (1)

Prompt OS)]: interaction of initial partons

Parton in-medium-modification imprinted in the final hadronic state

Prompt photons are not perturbed by the medium

Medium-induced OS)]: multiple scattering of final-

state partons


Photon sources (2)Photon sources (2)Thermal photons from QGP:

Photons from HG: , , 0

Photon rates from QGP and HG might be similar, but can be distinguishable due to different space-time evolution


Contributions to direct photon Contributions to direct photon spectrumspectrum


Photon predictions Photon predictions for for p+p at LHCp+p at LHC

15 counts15 counts

3535

Start up scenario :

2 PHOS modules

(=40, y=0.25)

L=1030 cm-2s-1

T=10 days=8.6105 s

LT= 8.6 108 mb-1

3·103·1066 events/GeV at 1 events/GeV at 1-2 -2 GeV/cGeV/c

115 events/GeV at 5 events/GeV at 3535 GeV/c GeV/c


00 p predictions redictions for p for p++p at LHCp at LHC

75 counts75 counts

5050

Start up scenario :

2 PHOS modules

(=40, y=0.25)

L=1030 cm-2s-1

T=10 days=8.6105 s

LT= 8.6 108 mb-1

3·103·1088 events/GeV at 1 events/GeV at 1-2-2 GeV/cGeV/c



Decay Decay vs Direct vs Direct

p+p collisions:p+p collisions: 00 dominates at all p dominates at all pTT

A+A collisions:A+A collisions: Jet quenching suppress Jet quenching suppress 00 RHIC:RHIC:

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

LHC:LHC: NN > N > N for p for pT T > 100 > 100

GeV/GeV/cc

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


00 nuclear modification factor nuclear modification factor

RRAAAA for the 5% most for the 5% most central Pb+Pb central Pb+Pb collisions at collisions at ssNNNN = = 5.55.5AA TeV with respect TeV with respect to p+p system.to p+p system.

Points obtained with Points obtained with NLO pQCD NLO pQCD approximation approximation

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


Photon Photon ppredictions redictions for for Pb+PbPb+Pb at LHC at LHC

50 counts50 counts

3535

Start up scenario :

2 PHOS modules

(=40, y=0.25)

L=1026 cm-2s-1

T=10 days=8.6105 s

LT= 8.6 104 mb-1




00 p predictions redictions for Pbfor Pb+Pb+Pb at LHC at LHC

40 counts40 counts

5050

Start up scenario :

2 PHOS modules

(=40, y=0.25)

L=1026 cm-2s-1

T=10 days=8.6105 s

LT= 8.6 104 mb-1


40 events/GeV at 50 GeV/c40 events/GeV at 50 GeV/c


Particle identification in PHOSParticle identification in PHOS

Photon are identified by 4 criteria:Photon are identified by 4 criteria:

1.1. Shape of the showers (e.m. or hadronic)Shape of the showers (e.m. or hadronic)

2.2. Charged particle matching by CPV detectorCharged particle matching by CPV detector

3.3. Time of flight measured by FEETime of flight measured by FEE

4.4. Photon isolationPhoton isolation


Photon efficiency and hadron Photon efficiency and hadron contaminationcontamination

Few % co

ntam

inatio

n

Single Hadron contaminationCentral Pb+Pb collisions

Single + Pb-Pb


Direct photons: experimental methodsDirect photons: experimental methods

Direct photons spectrum at low pDirect photons spectrum at low pTT can be can be measured statistically:measured statistically:

Raw spectrum of reconstructed and Raw spectrum of reconstructed and identified photons is accumulatedidentified photons is accumulated

This raw spectrum is to be corrected forThis raw spectrum is to be corrected for hadron contaminationhadron contamination photon conversionphoton conversion reconstruction and identification efficienciesreconstruction and identification efficiencies geometrical acceptancegeometrical acceptance


Experimental methods (cont’d)Experimental methods (cont’d)

Decay photon spectrum is to be reconstructed:Decay photon spectrum is to be reconstructed: 00 spectrum is reconstructed by 2-photon invariant spectrum is reconstructed by 2-photon invariant

mass distributions for each pmass distributions for each pTT-bin-bin Combinatorial background is evaluated by event-Combinatorial background is evaluated by event-

mixing techniquesmixing techniques 00 spectrum is corrected for spectrum is corrected for

reconstruction efficiencyreconstruction efficiency photon conversionphoton conversion geometrical acceptancegeometrical acceptance

Similar procedures should be done for Similar procedures should be done for and other and other neutral mesons if possible, heavy mesons neutral mesons if possible, heavy mesons contribution is evaluated by mcontribution is evaluated by mTT scaling scaling

Reconstructed neutral meson spectra should be put Reconstructed neutral meson spectra should be put into simulation to produce decay photon spectrainto simulation to produce decay photon spectra


00 detection in pp via 2 detection in pp via 2 inv.mass inv.mass

Practically no combinatorial background at pT>1 GeV/c

1 GeV/c 2 GeV/c 3 GeV/c 4 GeV/c

5 GeV/c 6 GeV/c 7 GeV/c 8 GeV/c


0 becomes visible over combinatorial background at pT>5 GeV/c

00 detection in Pb+Pb via 2 detection in Pb+Pb via 2 inv.mass inv.mass

1 GeV/c 2 GeV/c

3 GeV/c 4 GeV/c

5 GeV/c

6 GeV/c 7 GeV/c

8 GeV/c 9 GeV/c


00 reconstruction in central Pb-Pb reconstruction in central Pb-Pb collisions: collisions: -mass spectrum-mass spectrum

S1: real -pairs

Invariant mass spectrum of photon pairs has too high combinatorial background at low pT which obscures the 0 peak

Example: -mass at pT=1 GeV/c in 200,000 central Pb-Pb collisions in PHOS

Number of combinations: N(N-1)/2

0


-mass spectrum in mixed events-mass spectrum in mixed events

S2: -pairs in mixed events

Combinatorial background can be constructed from totally uncorrelated photons from different events

Example: -mass at pT=1 GeV/c of all combinations from 10 consequent events

Number of combinations: 10N2


Combinatorial background Combinatorial background normalizationnormalization

R=S1/S2: normalization

Normalization of real-pair spectrum and mixing-event pair spectrum can be obtained in the mass region of uncorrelated pairs

(200<M<400 MeV/c2)


Combinatorial background subtractionCombinatorial background subtraction

S3=S1-R×S2

After subtraction the 0 peak is revealed above almost zero background

But:

•residual correlated pairs are observed at low M which gives a background for photon interferometry studies (see later)

•Statistical errors are higher due to subtraction of two large numbers


00 detection in central Pb-Pb collisions detection in central Pb-Pb collisions

200,000 central Pb-Pb collisions (b<2 fm)

4 minutes of the LHC run at the nominal luminosity

L=51026 cm-2s-1


Direct photons at high pDirect photons at high pTT

1

1

1

1

50 GeV 70 GeV

90 GeV 110 GeV

0 0

00

Direct photons at high pDirect photons at high pTT can be identified E-by-Ecan be identified E-by-Eby the shape of the by the shape of the showershower


Direct photons at high pDirect photons at high pTT::efficiency and contaminationefficiency and contamination

ppTT = 90 GeV/c: = 90 GeV/c:

P(P(,,) = 60 %) = 60 %P(P(,,00) = 5%) = 5%


Direct photons:Direct photons:sources of systematical errorssources of systematical errors

Errors in inclusive photon spectrumErrors in inclusive photon spectrum Errors in decay photon spectrumErrors in decay photon spectrum Errors in theoretical assumptions on backgroundErrors in theoretical assumptions on background

ErrorError ppTT=1.5 =1.5 GeV/cGeV/c

ppTT=5 GeV/c=5 GeV/c

detectiondetection 3%3% 3%3%

00 detection detection 5%5% 5%5%

detectiondetection 3%3% 3%3%

Hadron Hadron contaminationcontamination

2.5%2.5% 1%1%

Non-vertex Non-vertex backgroundbackground

2%2% 1%1%TotalTotal 7.3%7.3% 6.7%6.7%


Expected direct photon excess over decay Expected direct photon excess over decay photonsphotons


Photon interferometryPhoton interferometry

1/R

С2

1

q

2/,

1),(

),(1

)()(

),(),(

2121

24

24

21

21212

22

kkKkkq

eKxxSd

eKxxSd

kPkP

kkPkkC Rq

iqx


CC22 in Pb-Pb at 158 AGeV/c (WA98) in Pb-Pb at 158 AGeV/c (WA98)


Photon correlations:Photon correlations:sources of systematical errorssources of systematical errors

Systematic errors in photon HBT extration.Systematic errors in photon HBT extration. Apparatus effects: close cluster interference, Apparatus effects: close cluster interference,

cluster merging andcluster merging and sp splittinglitting Photon spectrum contamination and admixture of Photon spectrum contamination and admixture of

the hadron (electron)the hadron (electron) correlationscorrelations Background Background photon photon correlations due tocorrelations due to

Residual correlation from Residual correlation from Bose-EinsteinBose-Einstein correlated correlated hadrons (hadrons (0000))

Resonance decays and conversion on material in front of Resonance decays and conversion on material in front of PHOSPHOS

Residual correlations from cResidual correlations from collective flow, jets, etc.ollective flow, jets, etc. Experimental rExperimental resolution ofesolution of the the relative momentum relative momentum


Two-photon correlations:Two-photon correlations:HIJING eventHIJING event

Comparison of two-photon correlation function of primary photons and reconstructed one as well as decomposition of correlations due to decays of heavy resonances.

Each contribution is shifted for clarity (all they go to zero at qinv > 30 MeV/c)

Both apparatus effects and background correlations are negligible at qinv>30 MeV/c

KT= 200 MeV


SummarySummary ALICE PHOS can measure direct photons and ALICE PHOS can measure direct photons and 00 in in

pp and PbPb collisions with reasonable statistics pp and PbPb collisions with reasonable statistics during first 10 days up to pduring first 10 days up to pTT=30-50 GeV/c.=30-50 GeV/c.

PHOS is able to measure thermal photons with the PHOS is able to measure thermal photons with the uncertainties better than 10%uncertainties better than 10% Main uncertainties come from photon identification Main uncertainties come from photon identification

efficiency and decay photon reconstruction.efficiency and decay photon reconstruction. PHOS can measure prompt photons at high pPHOS can measure prompt photons at high pTT with with

the uncertainties better then a few %the uncertainties better then a few % Main contamination is due to Main contamination is due to 00 misidentification misidentification

PHOS provides an opportunity to PHOS provides an opportunity to measure two-measure two-photon correlations, and in particular, direct photon photon correlations, and in particular, direct photon HBT correlationsHBT correlations Background correlations and apparatus effects distort two-Background correlations and apparatus effects distort two-

photon correlation function at q<20-30 MeV/c.photon correlation function at q<20-30 MeV/c.

Documents

Direct photon detection in pp and PbPb collisions in the ALICE experiment at LHC