Photon Physics at LHC-ALICEJSPS Research Fellow / University of Tsukuba

T. HoraguchiOct. 15 2009 for HAWAII2009

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OutlineIntroductionPhoton PhysicsLow pT PhotonVirtual Photon MeasurementLHC ALICE ExperimentElectron Identification with TRDInvariant Mass SpectrumEvaluation of Statistics for LHC First YearSummary & Future Plan

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What dose mean the measurement of direct photons ?

Direct photons in p+p collisionsTest of pQCD calculationObtain the gluon distribution functionReference data of the heavy ion

collisionsDirect photons in heavy ion collisions

Jet quenchingThermal photons

Direct photons are a clear probe to investigate the characteristics of evolution of the matter created by heavy ion collisions.

Penetrate the created matter without the strong interaction

Emitted from every stage of collisions Hard photons (High pT)

– Initial hard scattering, Pre-equilibrium

Thermal photons (Low pT)– Carry the thermodynamic

information from QGP and hadron gas

Introduction

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Direct Photon Measurement in ALICE

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Hard photon Strong suppression of high pT

hadrons will help to improve the S/N ratio

High pT photons can be found Thermal photon

Direct evidence of thermal equilibration

Created matter in LHC will have high temperature, high density and long life time matter comparison with RHIC, so we can expect large thermal photon component in ALICE

Primary contributor in low pT regionThermal photon measurement is

very challenging because it is very hard due to a large background from hadron decays.

Low pT Photons

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In ‘real’ photon measurement Measured yield with a large systematic errorDifficulty on measuring low pT “real” direct photons

1. Finite energy resolution of the EMCal

2. Large hadron background

Advantages on measuring ‘virtual’ photons

1. High momentum resolution of the TPC

2. Reliable estimation of the hadron decay components using Kroll-Wada formula

Experimental determination is very important since applicability of pQCD is doubtable in low pT region

Virtual Photon Measurement

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Any source of real g can emit g* with very low mass. Convert direct g* fraction to real direct photon yield

S : Process dependent factor

qg*

g q

e+

e-

g SdN

MMm

Mm

dMNd

eeee

e

ee

e

ee

1214132

2

2

2

22

*

*

inclusive

direct

inclusive

direct

gg

gg

Kroll-Wada formula

Possible to separate hadron decay components from virtual photon in the proper mass window.

LHC ALICE Experiment

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• TPC (Time Projection Chamber)• Main tracking device

– |h| < 0.9, full azimuth• Largest ever

– 88 m3, 10 m long, 5.6 m diameter, 570 k channels

– 3 % X0, Ne (86)/CO2 (9.5)/ N2 (4.5), O2 ~ 1 ppm

– max. 80 MB/event (after compression)

– ITS(Inner Tracking System)– Tracking (|h|< 1) + multiplicity (|h|

< 2)– Si pixel/drift/strip; 2 layers each rf

resolution: 12, 38– TRD(Transition Radiation Detector)

– Tracking and particle identification– |h| < 0.9, full azimuth– 400 – 600 mm resolution in rf, 23

mm in z– e/ separation > 100 at pT > 3

GeV/c– Track finding efficiency ~ 90 % @ pT >

1GeV/c– Momentum resolution of electrons ~ 2%

@ pT > 4GeV/c

ALICE

CMS

LHC-b

ATLAS

• LHC can accelerate up to• 14 TeV p+p collisions• 5.5 TeV Pb+Pb collisions

• In first year , 7TeV pp collisions will run from this November !

Electron ID with TRD (1)

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Used the production of ALICE full detector simulation with PYTHIA .

The fraction of electron (material conversion or hadron decay) increase with increasing TRD layer.

TRD 1 TRD 2 TRD 3 TRD 4 TRD 5 TRD 6

Blue : pionGleen: material conversionRed : hadron decay

pT(GeV/c)

Electron ID with TRD (2)

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The “efficiency x purity” is the highest with more than 4 layers of TRD, so we decided to apply TRD 4 layers cut in current analysis.

Magenta : purityBlue : efficiency

Red : efficiency x purity

Invariant Mass Spectrum

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Combinatorial background and Conversion electron pair dominates in the invariant mass spectrum.

Total mass yield is almost described by the combinatorial and material conversion background within the statistical error. But it indicates to need more statistics and analysis is ongoing.

Evaluation the Statistics in First Year

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Red : 100M eventBlue : 1G event

Evaluation from NLO pQCD calculation Used INCNLO

http://wwwlapp.in2p3.fr/lapth/PHOX_FAMILY/readme_inc.htm

CTEQ6M, BFG √s : 7TeV pp μ : 0.5pT,1.0pT,2.0pT

Evaluation of the number of the virtual photon Error propagation of

background subtraction included.

Required Trigger : MB Assumed DAQ

rate :100Hz & Duty factor : ~25%

100M event ~ 2 Month 1G event ~ 20 Month Measured pT will reach

~5GeV/c

Summary & Future PlanALICE at LHC starting in month !Photon Physics at LHC-ALICE is important

p+p collisions : Test of pQCD calculationPb+Pb collisions : Measurement of thermal photons

Preparation for low pT photons measurement @ALICEUsing the direct photon measurement via internal

conversion methodWorking Group for this analysis was established.Precise study in more statistics is ongoing at GRID!

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Backup Slides

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Combinatorial BackgroundCombinatorial background is

evaluated using mixed event method.

Normalization is done using the like sign pair.

The normalized combinatorial background is good agreement with the unlike sign pair in high mass region.

Black : unlike sign pairRed : Like sign pair (++)Blue : Like sign pair (--)

Black : unlike sign pairRed : Normalized combinatorialbackground

NNN 2

)cos1(2 eeee

comee PPM

e-’

e+e-e+’

Combinatorial pair

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Photon Physics : Thermal PhotonsRHIC outcome

radiation at 300 – 500 MeV implied indirect measurement via g* cf. critical temperature ~ 170 MeV

models not strongly constrained

LHC prospectdirect measurement of thermal photons

higher temperature + longer life time reduced background due to quenching ALICE-PHOS detector

understanding of thermal properties of partonic system

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Background Sources

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Real signal di-electron continuum

Background sources1. Combinatorial background2. Material conversion pairs3. Additional correlated

background– Cross pairs from decays

with 4 electrons in the final state

– Pairs in same jet or back-to-back jets

Hadron decays 0, h, h’, w, f, r, J/y, y’

π0

π0

e+e-

e+

e-γ

γ

π0e-γ

e+

π0 γ

e+e-

e-

e+

Jet cross pair

Dalitz + conversion cross pair

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Time Projection ChamberMain tracking device

|h| < 0.9, full azimuthLargest ever

88 m3, 10 m long, 5.6 m diameter, 570 k channels

3 % X0, Ne (86)/CO2 (9.5)/ N2 (4.5), O2 ~ 1 ppmmax. 80 MB/event (after compression)

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Inner Tracking SystemTracking (|h|< 1) + multiplicity (|h|< 2)Si pixel/drift/strip; 2 layers each

rf resolution: 12, 38, 20 mm

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Transition Radiation DetectorTracking and particle identification

|h| < 0.9, full azimuth400 – 600 mm resolution in rf, 23 mm in ze/ separation > 100 at pT > 3 GeV/c

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