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Direct photon detection in pp and PbPb collisions in the ALICE experiment at LHC. Yuri Kharlov For the ALICE PHOS collaboration 5 th International Conference on Perspectives in Hadronic Physics 22-26 May 2006. Outline. ALICE photon detectors Physics motivation Photon sources

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direct photon detection in pp and pbpb collisions in the alice experiment at lhc
Direct photon detection in pp and PbPb collisions in the ALICE experiment at LHC

Yuri Kharlov

For the ALICE PHOS collaboration

5th International Conference on Perspectives in Hadronic Physics

22-26 May 2006

outline
Outline
  • ALICE photon detectors
  • Physics motivation
  • Photon sources
  • Expected rates
  • Experimental methods

Direct photons in ALICE

alice experiment at lhc
ALICE experiment at LHC

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

Direct photons in ALICE

alice setup
ALICE setup

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

Direct photons in ALICE

photon detectors in alice
Photon detectors in ALICE

EMCAL

See a talk by N.Bianchi on 23 May

PHOS

Direct photons in ALICE

phos crystals
PHOS 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=2.16

size: 222216 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

Direct photons in ALICE

main physics interests
Main physics interests
  • Photon are highly penetrating particles which escape from the hot nuclear matter almost intact and, therefore, carry undistorted information from any stage of the nuclear matter evolution
  • Direct photons are produced in elementary interactions acts in the nuclear matter:
    • in hard QCD processes, observed mainly at high pT;
    • In emission of thermalized nuclear matter, observed mainly at low pT

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

  • Fragmentation photons
  • Decay photons reveal medium-induced modifications of hadron properties.
  • Interferometry of photons can be used as a tool to measure geometrical size of the source.

See a talk by F.Arleo on 24 May

Direct photons in ALICE

photon sources 1

Prompt g [O(aS)]: interaction of initial partons

  • Parton in-medium-modification imprinted in the final hadronic state
  • Prompt photons are not perturbed by the medium

Medium-induced g [O(a2S)]:multiple scattering of final-state partons

Photon sources (1)

Direct photons in ALICE

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

Photon sources (2)

Direct photons in ALICE

photon predictions for p p at lhc
Photon predictions for p+p at LHC

Start up scenario :

2 PHOS modules

(Df=40, Dy=0.25)

L=1030 cm-2s-1

T=10 days=8.6105 s

LT= 8.6 108 mb-1

3·106 events/GeV at 1-2 GeV/c

15 events/GeV at 35 GeV/c

15 counts

35

Direct photons in ALICE

p 0 p redictions for p p at lhc
p0 predictions for p+p at LHC

Start up scenario :

2 PHOS modules

(Df=40, Dy=0.25)

L=1030 cm-2s-1

T=10 days=8.6105 s

LT= 8.6 108 mb-1

3·108 events/GeV at 1-2 GeV/c

75 events/GeV at 50 GeV/c

75 counts

50

Direct photons in ALICE

decay g vs direct g
Decay g vs Direct g

p+p collisions:

  • 0 dominates at all pT

A+A collisions:

  • Jet quenching suppress 0
  • RHIC:
    • Ng > Np for pT > 10 GeV/c
  • LHC:
    • Ng > Np for pT > 100 GeV/c

Photon Yellow Report hep-ph/0311131

Direct photons in ALICE

p 0 nuclear modification factor
p0 nuclear modification factor
  • RAA for the 5% most central Pb+Pb collisions at sNN = 5.5A TeV with respect to p+p system.
  • Points obtained with NLO pQCD approximation

Photon Yellow Report hep-ph/0311131

Direct photons in ALICE

photon p redictions for pb pb at lhc
Photon predictions for Pb+Pb at LHC

Start up scenario :

2 PHOS modules

(Df=40, Dy=0.25)

L=1026 cm-2s-1

T=10 days=8.6105 s

LT= 8.6 104 mb-1

7·107 events/GeV at 1-2 GeV/c

50 events/GeV at 35 GeV/c

50 counts

35

Direct photons in ALICE

p 0 p redictions for pb pb at lhc
p0 predictions for Pb+Pb at LHC

Start up scenario :

2 PHOS modules

(Df=40, Dy=0.25)

L=1026 cm-2s-1

T=10 days=8.6105 s

LT= 8.6 104 mb-1

2·109 events/GeV at 1-2 GeV/c

40 events/GeV at 50 GeV/c

40 counts

50

Direct photons in ALICE

particle identification in phos
Particle identification in PHOS

Photon are identified by 4 criteria:

  • Shape of the showers (e.m. or hadronic)
  • Charged particle matching by CPV detector
  • Time of flight measured by FEE
  • Photon isolation

Direct photons in ALICE

photon efficiency and hadron contamination
Photon efficiency and hadron contamination

Single g

Central Pb+Pb collisions

Hadron contamination

Single g + Pb-Pb

Few % contamination

Direct photons in ALICE

direct photons experimental methods
Direct photons: experimental methods

Direct photons spectrum at low pT can be measured statistically:

  • Raw spectrum of reconstructed and identified photons is accumulated
  • This raw spectrum is to be corrected for
    • hadron contamination
    • photon conversion
    • reconstruction and identification efficiencies
    • geometrical acceptance

Direct photons in ALICE

experimental methods cont d
Experimental methods (cont’d)

Decay photon spectrum is to be reconstructed:

  • 0 spectrum is reconstructed by 2-photon invariant mass distributions for each pT-bin
  • Combinatorial background is evaluated by event-mixing techniques
  • 0 spectrum is corrected for
    • reconstruction efficiency
    • photon conversion
    • geometrical acceptance
  • Similar procedures should be done for  and other neutral mesons if possible, heavy mesons contribution is evaluated by mT scaling
  • Reconstructed neutral meson spectra should be put into simulation to produce decay photon spectra

Direct photons in ALICE

0 detection in pp via 2 inv mass
0 detection in pp via 2 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

Practically no combinatorial background at pT>1 GeV/c

Direct photons in ALICE

0 detection in pb pb via 2 inv mass
0 detection in Pb+Pb via 2 inv.mass

1 GeV/c

2 GeV/c

5 GeV/c

3 GeV/c

4 GeV/c

6 GeV/c

7 GeV/c

8 GeV/c

9 GeV/c

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

Direct photons in ALICE

p 0 reconstruction in central pb pb collisions gg mass spectrum
p0 reconstruction in central Pb-Pb collisions: gg-mass spectrum

Invariant mass spectrum of photon pairs has too high combinatorial background at low pT which obscures the p0 peak

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

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

S1: real gg-pairs

p0

Direct photons in ALICE

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

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

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

Number of combinations: 10N2

S2: gg-pairs in mixed events

Direct photons in ALICE

combinatorial background normalization
Combinatorial background normalization

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<Mgg<400 MeV/c2)

Direct photons in ALICE

combinatorial background subtraction
Combinatorial background subtraction
  • After subtraction the p0 peak is revealed above almost zero background
  • But:
  • residual correlated pairs are observed at low Mgg which gives a background for photon interferometry studies (see later)
  • Statistical errors are higher due to subtraction of two large numbers

S3=S1-R×S2

Direct photons in ALICE

p 0 detection in central pb pb collisions
p0 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=51026 cm-2s-1

Direct photons in ALICE

direct photons at high p t

50 GeV

70 GeV

l21

l21

90 GeV

110 GeV

l21

l21

Direct photons at high pT

Direct photons at high pT can be identified E-by-E

by the shape of the shower

g

g

p0

p0

g

p0

g

p0

Direct photons in ALICE

direct photons at high p t efficiency and contamination
Direct photons at high pT:efficiency and contamination

pT = 90 GeV/c:

P(g,g) = 60 %

P(g,p0) = 5%

Direct photons in ALICE

direct photons sources of systematical errors
Direct photons:sources of systematical errors
  • Errors in inclusive photon spectrum
  • Errors in decay photon spectrum
  • Errors in theoretical assumptions on background

Direct photons in ALICE

photon interferometry
Photon interferometry

С2

l

1/R

1

q

Direct photons in ALICE

c 2 in pb pb at 158 agev c wa98
C2 in Pb-Pb at 158 AGeV/c (WA98)

Direct photons in ALICE

photon correlations sources of systematical errors
Photon correlations:sources of systematical errors

Systematic errors in photon HBT extration.

  • Apparatus effects: close cluster interference, cluster merging and splitting
  • Photon spectrum contamination and admixture of the hadron (electron)correlations
  • Background photon correlations due to
    • Residual correlation from Bose-Einstein correlated hadrons (00)
    • Resonance decays and conversion on material in front of PHOS
    • Residual correlations from collective flow, jets, etc.
  • Experimental resolution of the relative momentum

Direct photons in ALICE

two photon correlations hijing event
Two-photon correlations:HIJING 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)

KT= 200 MeV

Both apparatus effects and

background correlations

are negligible at qinv>30 MeV/c

Direct photons in ALICE

summary
Summary
  • ALICE PHOS can measure direct photons and 0 in pp and PbPb collisions with reasonable statistics during first 10 days up to pT=30-50 GeV/c.
  • PHOS is able to measure thermal photons with the uncertainties better than 10%
    • Main uncertainties come from photon identification efficiency and decay photon reconstruction.
  • PHOS can measure prompt photons at high pT with the uncertainties better then a few %
    • Main contamination is due to 0 misidentification
  • PHOS provides an opportunity to measure two-photon correlations, and in particular, direct photon HBT correlations
    • Background correlations and apparatus effects distort two-photon correlation function at q<20-30 MeV/c.

Direct photons in ALICE

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