1 Prompt Photon Production from Proton - proton Collisions at √s = 62.4 GeV in PHENIX ( PHENIX 実験における重心系 62.4 GeV での陽子 - 陽子衝突からの 直接光子の生成断面積の測定 ) JPS meeting March 26th, pZF-11 Kohichi Sakashita ( Tokyo Tech ) for the PHENIX Collaboration

2 Contents 1.Introduction 2.PHENIX detector and data set 3.Method of prompt photon measurement 4.Result 5.Summary

Introduction Production of prompt photon –Quark - gluon scattering is dominant sub-process at pp collision in  √ s = 62.4 GeV The related experiment –PHENIX √ s = 200 GeV –R806, AFS, CCOR and CMOR using ISR collider at √ s = 63 GeV in CERN Test the applicability of perturbative QCD (pQCD) –Comparing the cross section of measurement to the one of pQCD calculation pQCD calculation in qg scattering : –Once the applicable range of pQCD is determined, the framework of pQCD can be used to calculate other quantities of interest, in particular A LL prompt photon proton gluon quark proton q(x), g(x) : PDF for quark, gluon : sub-process cross section

Introduction Double helisity asymmetry ( A LL ) Comparing to A LL in 200 GeV, large Bjorken’s x can be reached at 62.4 GeV x T √s = 200 GeV

5 Proton beam 2. PHENIX Detector and Data Set PHENIX central arm detector –  = 90° x 2, |  | < 0.35 Data set –2006 pp run –Integrated luminosity : pb -1 Basic analysis cuts EMCal&BBC trigger Vertex cut |z| < 30 cm Remove 2 edge towers, dead and hot towers –Event selection p T > 2 GeV/c Shower shape cut Charge veto with PC3 prompt 

Method of Prompt Photon Measurement Main issue of prompt photon measurement –Evaluation of systematic uncertainties  0 extraction and so on –Prompt photon yields is small signal of all photon About 10 % at 3 GeV/c

Method of Prompt Photon Measurement N prompt  = N all  - (1+A)*(1+R)*N   tag By measuring N all   and N   tag, one can extract small N prompt  signal (   tagging method ) N   tag N prompt  N all  x R* N   tag x A*(1+R)* N   tag  ’     decay photon –The ratio of  ’and  0 production to   production –The ratio of branching ratio of photon of  ’and  0 to the branching ratio of photon of   A =  σ i σ   Br i  / Br  ->   i :  ’      decay photon –Detecting two photon ( N   tag ) Reconstruction invariant mass –Missing one photon Evaluated by fast MC simulation The ratio ( R ) of the missing one photon to detecting two photon

Result - cross section Cross section : pQCD calculation with NLO and CTEQ6M PDF agrees with experiment within theoretical uncertainty and experimental uncertainty

Result - comparison of PHENIX √s = 200 GeV and ISR experiments Results of ISR agree with this result within the experimental uncertainty Cross section slope at √s = 200 GeV is gentler than one at √s = 62.4 GeV Open black circle : PHENIX at √s = 200 GeV ( 2005 year ) The others : ISR experiments at √s = 63 GeV

10 5. Summary Test the applicability of pQCD calculation Data set –pp collision at 2006 year –√ s = 62.4 GeV –Integrated luminosity : pb -1 Prompt photon yields as a function of p T are extracted by the  0 tagging method ( N prompt  = N all  - (1+A)*(1+R)*N   tag ) with PHENIX central arm detector (  = 90° x 2, |  | < 0.35 ) pQCD calculation with NLO and CTEQ6M PDF agrees with experiment within theoretical uncertainty and experimental uncertainty Results of ISR agree with this result within the experimental uncertainty

11 Back up

Method of Prompt Photon Measurement Main issue of prompt photon measurement –Evaluation of systematic uncertainties  0 extraction and so on –Prompt photon yields is small signal of all photon Prompt photon / All photon

13 4. Result - systematic errors Error in N   tag –Fit ( Gauss + pol.3 ) to the region of pi0 mass peak to extract  0 photon with 3  ( 105 < M  < 165MeV/c 2 ) –Difference of between N   tag with pol.2 and N   tag with pol.3 and between N   tag with 3  and N   tag with 4  is assigned as the error –3.4 % to the N   tag with pol.3 and 3  at 2 GeV/c 2.8 % to the N   tag with pol.3 and 3  at 3.75 GeV/c

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15 4. Result - systematic errors Neutral hadron contamination Secondary origin Error in 1+A Dalitz decay pi0 partner photon conversion loss Error in N   tag 1+R (acceptance) 1+R (Minimum E cut) 1+R (Pi0 cross section slope) Energy scale uncertainty Luminosity uncertainty Conversion error BBC bias uncertainty W = prompt / inclusiveError of C is scaled by 1/W - 1

16 x xDG(x) present x-range GS-C GS-C, ΔG = 1