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Measurement of prompt photon in sqrt(s)=200GeV pp collisions

Kensuke Okada discusses the significance of prompt photons in probing parton structure, their role in hadron collisions, and their relevance at RHIC for QGP research and gluon spin measurement. Using PHENIX detector, data from Run3 proton-proton runs were analyzed for improving signal-to-noise ratios and cross-section calculations. The study includes rejection methods for non-photon backgrounds and Pi0 tagging efficiency to enhance prompt photon measurements.

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Measurement of prompt photon in sqrt(s)=200GeV pp collisions

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  1. Measurement of prompt photon in sqrt(s)=200GeV pp collisions Kensuke Okada (RIKEN-BNL research center) for the PHENIX collaborations 9/27,2004 JPS K.Okada

  2. Motivation • Physics meaning of prompt photon • Is a good probe of parton structure in proton. • One of simple process at hadron collisions. • Why prompt photon at RHIC? • A reference for QGP search • A baseline for measurement of gluon spin • RHIC provide the highest energy as proton-proton collisions in the world. K.Okada

  3. PHENIX detector Central arm (west) Rapidity |y|<0.35 EMCalwith good resolution (~10*10mrad2) DC for charged hadron veto Beam-beam counter (BBC) for trigger and vertex determination 2 [m] K.Okada

  4. Run3 proton proton run Beam acquisition : From April to May 2003 Beam : 100GeV proton-proton (sqrt(s)=200GeV) Data were taken with BBC,EMCal trigger 266nb-1 (corresponds to 5.450*109 events of BBC trigger) Plateau above 3GeV Black points: Data Red line: MC expectation K.Okada

  5. Experimental difficulty Contents of EMCal hits Photonic decay of hadrons (pi0,eta,etc) Hadronic interactions Direct photons Signal to noise ratio is roughly （pT dependent) S/N0.21 (pt 5 17GeV/c) To get prompt photon, we will subtract known backgrounds from EMCal clusters. K.Okada

  6. Improvement of Ｓ／Ｎ（１） (1) Non-photon background rejection Electromagnetic shower shape requirement Charged hadron veto using the drift chamber tracks. (2) Photon background rejection Pi0 tagging Other components are calculated based on tagged pi0 (MC). Pi0 tagging efficiency is essential !! K.Okada

  7. Electromagnetic shower shape cut For charged hadrons Integral from x to 1. EM shower probability With Prob>0.02 cut 40% of hadronic shower hits are rejected 98% of photons are remained K.Okada

  8. Pi0 tagging Reason to fail Our efforts to recover them Assign edges to guard veto region (only for the partner search) Careful definition Set Emin at 150MeV (as low as possible). Charged veto with DC tracks, not with a detector in front of the EMCal In our pT region, those are rejected by the EM shower shape cut. • Out of EMCal Arm • EMCal bad area • Less than the minimum E • Photon conversions • Photon merging K.Okada

  9. pi0 photon missing ratio (with MC) Input: pi0 spectra, Energy resolution, Shower size from measurements The same MC used in pi0 cross section measurement. This figure shows components of photon from pi0. (A) 2 photons tagged as pi0 (B) 1photon (the partner missing) A Merged photons (to be discarded by the shower shape cut And those have almost double energy) B Photon from pi0 Miss/ tag (=B/A) 40% at 5GeV/c 20% at 10GeV/c K.Okada

  10. Contributions of eta, omega, etc. It comes from a product of (production ratio) and (decay branching ratio) to pi0. (Eta2)/(pi0 2) 0.45 (=production ratio) * 0.394/0.988 (= Decay branching ratio) = 0.18 ( ,pi0)/(pi0 2) 0.8 *0.087/0.988 *1/2 = 0.034 etc We used the value below with error (non pi0 hadrons to gamma)/ (pi0 to gamma) = 0.23+-0.05 K.Okada

  11. Signal to noise ratio Data S:signals = all - (pi0, eta contributions) N:background = (all – signal – pi0tagged) With pi0 tagging ~ Raw A big improvement was obtained. K.Okada

  12. Cross section calculation Factors 1/Luminosity: 1/266nb-1 (=5.450e9 events/20.5mb) 1/bbc_bias: 1/0.785 1/(acceptance+smearing): 1/0.0982 1/(shower shape cut efficiency): 1/0.98 1/(Conversion probability): 1/0.97 K.Okada

  13. PHENIX preliminary Direct photon cross section This is the plot. Inclusive prompt photon Line: statistical error Box: systematic error K.Okada

  14. Systematic error sources Lowest Highest 5-5.5 [GeV/c] 15-17 [GeV/c] Pi0 photon missing ratio 30% 5 Non pi0 contribution 27 6 Photon acceptance and smearing 10 10 Photon conversion effect 1 1 Luminosity measurement 12 12 BBC trigger bias 3 3 Total 43% 18 Point to point global Errors on the backgrounds resulted errors on the signal enlarged especially at lower pT region. K.Okada

  15. PHENIX preliminary Gamma/pi0 ratio Ratio to pi0 fit function of our measurement in Run2 (Y=20.39*pT^-8.285) K.Okada

  16. Summary Pi0 tagging and subtraction method is developed : It improves the signal to noise ratio We measured prompt photon production cross section (pT>5GeV/c). This is the measurement from the highest energy pp collision ever done. The next speaker (Hisa) will apply an isolation cut to photons. It is expected to enhance the “direct” component of prompt photons. He will also summarize the comparison of our data with other experiments and NLO pQCD calculations. K.Okada

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