1. Feb. 6 2009High-pT Physics at LHC09 @ Prague1 T. Horaguchi Hiroshima University takuma@hepl.hiroshima-u.ac.jp Feb. 4 for the 4 th International Workshop High-pT Physics at LHC09 @ Prague

2.  Introduction  Direct Photon Measurement in ALICE  Low p T Photon  Virtual Photon Measurement  Background Study  Background Sources  Combinatorial Background  Hadron Decay  Combinatorial + Hadron  P T Sliced Mass Spectra  Evaluation the Statistics of First Year  Summary & Future Plan Feb. 6 2009High-pT Physics at LHC09 @ Prague2

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 Feb. 6 2009High-pT Physics at LHC09 @ Prague3/16

4. Feb. 6 2009High-pT Physics at LHC09 @ Prague4/16  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. Feb. 6 2009High-pT Physics at LHC09 @ Prague5/16 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 pT region

6. Feb. 6 2009High-pT Physics at LHC09 @ Prague6/16  Case of Hadrons Obviously S = 0 at M ee > M hadron  Case of  * – If pT 2 >>M ee 2  Possible to separate hadron decay components from virtual photon in the proper mass window.  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

7. Feb. 6 2009High-pT Physics at LHC09 @ Prague7  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

8. Feb. 6 2009High-pT Physics at LHC09 @ Prague8  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

9.  PYTHIA  MSEL=1  14TeV pp  10M event  4  coverage  No Detector Simulation  No virtual photon event Feb. 6 2009High-pT Physics at LHC09 @ Prague9 Pi+- Red :  + Blue :  -  distribution (  +    e + +e - from hadron decay P T distribution

10. Feb. 6 2009High-pT Physics at LHC09 @ Prague10  The hadron decay background affects mainly in low mass region. Hadron Decay Mode

11.  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. Feb. 6 2009High-pT Physics at LHC09 @ Prague11 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

12.  The combinatorial + hadron decay background is very good agreement with mass spectrum !  The mass spectrum in Low mass region is mainly produced by hadron decay.  The mass spectrum in High mass region is mainly produced by combinatorial background.  The contribution of jet correlated pair seems to be negligible in this mass region. Feb. 6 2009High-pT Physics at LHC09 @ Prague12 Black: Total unlike sign pair Red : Combinatorial + Hadron Decay Good Agreement !

13.  The combinatorial + hadron decay background is very good agreement with mass spectra for each p T bin ! Feb. 6 2009High-pT Physics at LHC09 @ Prague13 0<pT<0.5GeV/c 3.5<pT<5.5GeV/c 2.0<pT<3.5GeV/c 1.0<pT<2.0GeV/c 0.5<pT<1.0GeV/c Total

14.  Evaluation from NLO pQCD calculation  Used INCNLO  http://wwwlapp.in2p3.fr /lapth/PHOX_FAMILY/ readme_inc.htm http://wwwlapp.in2p3.fr /lapth/PHOX_FAMILY/ readme_inc.htm  CTEQ6M, BFG  √s : 14TeV pp  μ : 0.5p T,1.0p T,2.0p T  Evaluation of the number of the virtual photon  Assumed DAQ rate :100KHz  1 Day ： ~2M  1 Month ： ~60M  3 Month: ~ 180M  Acceptance Correction  Considered TRD acceptance  |  |< 0.9   coverage: 8/18 x 2  Feb. 6 2009High-pT Physics at LHC09 @ Prague14 30 Days 90 Days Enough Statics !

15.  The e + e - pair mass spectrum is calculated in p+p 14TeV collisions with PYTHIA simulation.  The mass spectrum is very good agreement with the combinatorial background + hadron decay background !  More precise background study will be needed with the detector simulation.  The statistics of virtual photon is evaluated for first year. The signal will be seen if we have more one month beam time. This result strongly encourages us to measure the direct photon via internal conversion ! Feb. 6 2009High-pT Physics at LHC09 @ Prague15

16. Feb. 6 2009High-pT Physics at LHC09 @ Prague16

17.  There are two processes:  “Direct process”  “Fragmentation process”  The direct photon production dominates two leading –order subprocesses  Quark-gluon Compton scattering (qg→  q)  Quark-anti-quark annhilation (qq- bar→  g) Feb. 6 2009High-pT Physics at LHC09 @ Prague17 Quark-gluon Compton scattering (qg→gq) at LO Quark-gluon Compton scattering (qg→gq) at LO Quark-anti-quark annihilation (qq-bar→ gg) at LO Quark-anti-quark annihilation (qq-bar→ gg) at LO Fragmentation Process

18.  The measurement of direct photon cross sections in proton-proton collisions  Midrapidity  varying from 19.4GeV to 63GeV was performed by  E706 at Tevatron.  E704 at FNAL.  UA6, WA70 and NA24 at SPS.  R110, R806 and R807 at ISR of CERN Feb. 6 2009High-pT Physics at LHC09 @ Prague18  The measurement of direct photon cross sections proton-anti-proton collisions  Midrapidity  varying from 24.3GeV to 1800GeV was performed by  UA6 at SPS.  UA1 and UA2 at  CDF and D0 at Tevatron.

19. Feb. 6 2009High-pT Physics at LHC09 @ Prague19