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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 - PowerPoint PPT Presentation
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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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 33
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.
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 44
ALICE setupALICE setup
On ALICE physics see a talk by M.Monteno on 24 May
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 55
Photon detectors in ALICEPhoton detectors in ALICE
PHOS
EMCAL
See a talk by N.Bianchi on 23 May
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 66
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 77
First PHOS module is assembled in First PHOS module is assembled in April-May 2006April-May 2006
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 88
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 99
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 1010
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 1111
Contributions to direct photon Contributions to direct photon spectrumspectrum
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 1212
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 1313
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
775 events/GeV at 5 events/GeV at 5050 GeV/c GeV/c
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 1414
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 1515
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 1616
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
7·107·1077 events/GeV at 1 events/GeV at 1-2-2 GeV/cGeV/c
50 events/GeV at 350 events/GeV at 355 GeV/c GeV/c
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 1717
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
2·102·1099 events/GeV at 1 events/GeV at 1-2-2 GeV/cGeV/c
40 events/GeV at 50 GeV/c40 events/GeV at 50 GeV/c
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 1818
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 1919
Photon efficiency and hadron Photon efficiency and hadron contaminationcontamination
Few % co
ntam
inatio
n
Single Hadron contaminationCentral Pb+Pb collisions
Single + Pb-Pb
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 2020
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 2121
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 2222
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 2323
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 2424
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 2525
-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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 2626
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)
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 2727
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 2828
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 2929
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 3030
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%
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 3131
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%
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 3232
Expected direct photon excess over decay Expected direct photon excess over decay photonsphotons
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 3333
Photon interferometryPhoton interferometry
1/R
С2
1
q
2/,
1),(
),(1
)()(
),(),(
2121
24
24
21
21212
22
kkKkkq
eKxxSd
eKxxSd
kPkP
kkPkkC Rq
iqx
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 3434
CC22 in Pb-Pb at 158 AGeV/c (WA98) in Pb-Pb at 158 AGeV/c (WA98)
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 3535
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 3636
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
22-26 May 200622-26 May 2006 Direct photons in ALICEDirect photons in ALICE 3737
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.