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Measurement of Direct Photons in  s=200GeV p+p collisions

Measurement of Direct Photons in  s=200GeV p+p collisions. K.Okada (RBRC) For the PHENIX collaboration JPS/DNP meeting September 20, 2005. Motivations. g. q. Compton. g. q. Test of our theoretical understanding based on QCD Direct information on gluon distribution in the proton

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Measurement of Direct Photons in  s=200GeV p+p collisions

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  1. Measurement of Direct Photons in s=200GeV p+p collisions K.Okada (RBRC) For the PHENIX collaboration JPS/DNP meeting September 20, 2005 K.Okada (RBRC)

  2. Motivations g q Compton g q • Test of our theoretical understanding based on QCD • Direct information on gluon distribution in the proton With polarized beam at RHIC, it is a probe for gluon polarization • Reference for A+A collisions g q Annihilation q g 20~10% 80~90% Dominant K.Okada (RBRC)

  3. Data 2 [m] RHIC run3 p+p 2003 April-May s=200GeV Proton-proton collisions Luminosity= 240nb-1 RHIC-PHENIX detector Central Arm (West) (Rapidity |y|<0.35) Electromagnetic Calorimeter (EMCal) Photon detection High granularity (~10*10mrad2) Drift chamber (DC) Charged hadron veto Beam forward / backward (Rapidity 3.1<|y|<3.9) Beam-beam counter (BBC) Triggering and vertex determination Luminosity measurement BBC and EMCal Trigger for the data taking K.Okada (RBRC)

  4. Analysis Strategy Photon cluster selection • Photon shape cut (important to remove merged 0 clusters) • Charged veto with DC track • Timing cut 0 photon tag • Count photons with 0 partner • Estimate 0photons without partner • (it is only kinematics and geometrical issue) Other hadron to photon estimation (,, etc.) • Scale 0 photon contribution by their production and branching ratio The rest is our direct photon signal !! (Subtraction method) K.Okada (RBRC)

  5. Subtraction method   Systematic error sources pT region Low Mid High Luminosity Energy scale Non photon BG Pi0 tag Missing 0 Non 0 photon    Ed3/dp3         West arm only   pT K.Okada (RBRC)

  6. Subtraction method NLO pQCD (by W.Vogelsang) CTEQ6M PDF =1/2pT, pT, 2pT With both arms Preliminary points Ed3/dp3 • NLO pQCD calculation explains the • data well. • At low pT, the data show an excess. • with large systematic error • but may be soft physics contributions • as well K.Okada (RBRC)

  7. Isolation cut method : 2 goals g q Compton g q 1 • Check if our direct photon signal is isolated. • (with LO view, they are isolated) • A confirmation is done by applying the same isolation cut • on photons from 0 K.Okada (RBRC)

  8. Isolation cut method : 2 goals 2 Can we extract the contribution of direct production? NLO pQCD calculation by W. Vogelsang (p+p at s=200 GeV) direct brems. K.Okada (RBRC)

  9. Isolation cut with PHENIX R Eg 0.5 PHENIX arm =0.7 =/2 • Starting from isolated photons 0.1E > Econe (R=0.5rad) Econe : photon energy + charged particle momentum • For the estimation of hadron contribution, “Isolated 0 photon” is introduced They have 0 partner. They pass the isolation cut when the partner energy is excluded. K.Okada (RBRC)

  10. Isolation cut method Preliminary points Isolation cut 0.1*E > Econe(R=0.5rad) No correction for isolation cut efficiency was applied. Ed3/dp3 K.Okada (RBRC)

  11. Ratios Direct photon : isolation / subtraction Photon from 0 : isolated photon / all 1 Isolation cut 0.1*E > Econe(R=0.5rad) Photons from 0 is reduced by the isolation cut. Direct photons are clearly isolated at high pT region. K.Okada (RBRC)

  12. Iso/sub ratio with a theory calculation 2 Isolation cut 0.1*E > Econe(R=0.5rad) By M.Werlen, JETPHOX -.35<y<.35 =pT BFG set2, CTEQ6M By W.Vogelsang, R=0.4 =pT, CTEQ6M At high pT, theory predictions are consistent with the data. K.Okada (RBRC)

  13. Summary We measured direct photon cross section in s=200GeV p+p collisions. Two methods: subtraction and isolation cut • Subtraction method NLO pQCD calculation explains the data well. An excess in low pT region? • Isolation cut method It confirmed the reduction of photons from 0. Direct photon signal is isolated in high pT region. Level of the ratio of isolation cut method to subtraction method is same as the theory prediction. K.Okada (RBRC)

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