1. JSPS Research Fellow / University of Tsukuba T. Horaguchi Oct. 15 2009 for HAWAII2009 2009/10/15HAWAII 20091

2. Outline Introduction Photon Physics Low p T Photon Virtual Photon Measurement LHC ALICE Experiment Electron Identification with TRD Invariant Mass Spectrum Evaluation of Statistics for LHC First Year Summary & Future Plan 2009/10/15HAWAII 20092

3.  What dose mean the measurement of direct photons ?  Direct photons in p+p collisions  Test of pQCD calculation  Obtain the gluon distribution function  Reference data of the heavy ion collisions  Direct photons in heavy ion collisions  Jet quenching  Thermal 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 2009/10/15HAWAII 20093

4. Direct Photon Measurement in ALICE 2009/10/15HAWAII 20094/16  Hard photon Strong suppression of high pT hadrons will help to improve the S/N ratio High p T 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 p T region Thermal photon measurement is very challenging because it is very hard due to a large background from hadron decays.

5. Low pT Photons 2009/10/15HAWAII 20095 In ‘real’ photon measurement  Measured yield with a large systematic error Difficulty 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 p T region

6. Virtual Photon Measurement 2009/10/15HAWAII 20096  Any source of real  can emit  * with very low mass.  Convert direct  * fraction to real direct photon yield S : Process dependent factor q  g q e+e+ e-e- Kroll-Wada formula  Possible to separate hadron decay components from virtual photon in the proper mass window.

7. LHC ALICE Experiment 2009/10/15HAWAII 20097 TPC (Time Projection Chamber) Main tracking device – |  | < 0.9, full azimuth Largest ever – 88 m 3, 10 m long, 5.6 m diameter, 570 k channels – 3 % X 0, Ne (86)/CO 2 (9.5)/ N 2 (4.5), O 2 ~ 1 ppm – max. 80 MB/event (after compression) – ITS(Inner Tracking System) – Tracking (|  |< 1) + multiplicity (|  |< 2) – Si pixel/drift/strip; 2 layers each r  resolution: 12, 38 – TRD(Transition Radiation Detector) – Tracking and particle identification – |  | < 0.9, full azimuth – 400 – 600  m resolution in r , 23 mm in z – e/  separation > 100 at p T > 3 GeV/c – Track finding efficiency ~ 90 % @ p T > 1GeV/c – Momentum resolution of electrons ~ 2% @ p T > 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 !

8. Electron ID with TRD (1) 2009/10/15HAWAII 20098  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 1TRD 2TRD 3TRD 4TRD 5TRD 6 Blue : pion Gleen: material conversion Red : hadron decay pT(GeV/c)

9. Electron ID with TRD (2) 2009/10/15HAWAII 20099  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 : purity Blue : efficiency Red : efficiency x purity

10. Invariant Mass Spectrum 2009/10/15HAWAII 200910  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.

11. Evaluation the Statistics in First Year 2009/10/15HAWAII 200911 Red : 100M event Blue : 1G event  Evaluation from NLO pQCD calculation  Used INCNLO  http://wwwlapp.in2p3.fr/l apth/PHOX_FAMILY/rea dme_inc.htm http://wwwlapp.in2p3.fr/l apth/PHOX_FAMILY/rea dme_inc.htm  CTEQ6M, BFG  √s : 7TeV pp  μ : 0.5p T,1.0p T,2.0p T  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

12. Summary & Future Plan ALICE at LHC starting in month ! Photon Physics at LHC-ALICE is important p+p collisions : Test of pQCD calculation Pb+Pb collisions : Measurement of thermal photons Preparation for low p T photons measurement @ALICE Using the direct photon measurement via internal conversion method Working Group for this analysis was established. Precise study in more statistics is ongoing at GRID! 2009/10/15HAWAII 200912

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17. Combinatorial Background Combinatorial 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 pair Red : Like sign pair (++) Blue : Like sign pair (--) Black : unlike sign pair Red : Normalized combinatorial background e-’ e+e+ e- e+’ Combinatorial pair 2009/10/1517HAWAII 2009

18. Photon Physics : Thermal Photons RHIC outcome radiation at 300 – 500 MeV implied indirect measurement via  * cf. critical temperature ~ 170 MeV models not strongly constrained LHC prospect direct measurement of thermal photons higher temperature + longer life time reduced background due to quenching ALICE-PHOS detector understanding of thermal properties of partonic system 2009/10/15HAWAII 200918

19. Background Sources 2009/10/15HAWAII 200919  Real signal di-electron continuum  Background sources 1. Combinatorial background 2. Material conversion pairs 3. 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, ,  ’, , , , J/ ,  ’ π0π0 π0π0 e+e+ e-e- e+e+ e-e- γ γ π0π0 e-e- γ e+e+ π0π0 γ e+e+ e-e- e-e- e+e+ Jet cross pair Dalitz + conversion cross pair

20. Time Projection Chamber Main tracking device |  | < 0.9, full azimuth Largest ever 88 m 3, 10 m long, 5.6 m diameter, 570 k channels 3 % X 0, Ne (86)/CO 2 (9.5)/ N 2 (4.5), O 2 ~ 1 ppm max. 80 MB/event (after compression) 2009/10/15HAWAII 200920

21. Inner Tracking System Tracking (|  |< 1) + multiplicity (|  |< 2) Si pixel/drift/strip; 2 layers each r  resolution: 12, 38, 20  m 2009/10/15HAWAII 200921

22. Transition Radiation Detector Tracking and particle identification |  | < 0.9, full azimuth 400 – 600  m resolution in r , 23 mm in z e/  separation > 100 at p T > 3 GeV/c 2009/10/15HAWAII 200922