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PHENIX measurement on direct photon production

PHENIX measurement on direct photon production. Jiamin Jin Journal Club 07/12/2006. Outline. motivation direct photon in pp & AuAu (RUN2) -- hep-ex/0502006, nucl-ex/0503003 direct photon in AuAu (RUN4) extend to higher pT two analyses to improve on mid pT range

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PHENIX measurement on direct photon production

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  1. PHENIX measurement on direct photon production Jiamin Jin Journal Club 07/12/2006

  2. Outline • motivation • direct photon in pp & AuAu (RUN2) -- hep-ex/0502006, nucl-ex/0503003 • direct photon in AuAu (RUN4) • extend to higher pT • two analyses to improve on mid pT range • using stachastic cuts (David et al) • photon internal conversion (Y. Akiba) -- various ANs J. Jin - JClub

  3. What are direct photons? • @LO level, 3 channels: • Compton scatting g+qg+q • Annihilation q+qbarg+g • The latter is suppressed for p+p, because qbar is less probable than g in the proton • The parton-parton scattering with the scattered quark or gluon fragmenting to a g • @NLO level: • Bremstrahlung emission of g from the quarks undergoing hard scattering J. Jin - JClub

  4. photons not from hadronic decays • Experimental challenge to differentiate gdirect & gdecay • cocktail subtraction method • p0 tagging method • Isolation cuts • Direct photons != isolated photons • Fragmentation and bremstralung processes will produce photons with jets J. Jin - JClub

  5. Why direct photons? • p+p • Test on QCD • explore PDF of the gluon inside the proton • Reduce uncertainty on pQCD photons in AA • A+A • Photons don’t strong interact with medium • Hard photons • Allow test of Ncoll scaling for hard processes • Interpret high-pt hadron suppression at RHIC • Thermal photons • Carry information of early stage of collisions • Detect QGP via thermal photon radiation J. Jin - JClub

  6. Direct photons in p+p • 1st step: determine direct-photon-candidate hits • reject a EMCal cluster if: • form an invariant mass in the p0 or h range with other clusters • asymmetry cut • combinatorial background studied by embedding (small effect in p+p) • a factor of 2 rejection achieved for cluster pT>5GeV/c J. Jin - JClub

  7. Formula of differential cross section • N: # of direct-photon-candidates in a DpT&Dy bin • Creco: acceptance and efficiency correction (MC sim.) • Cconv: correction for photon conversions (<7.4%) • Closs: correction for the loss of true direct photons due to combinatorial bkgd (<2%) • f = 0.75(+/-0.02): MinBias trigger efficiency • L: integrated luminosity (~41nb-1 for RUN2 p+p) J. Jin - JClub

  8. 2nd step: deal with the remaining hadronic decay photons • mostly from missing the partner photon • also from other decay sources (w,h’,…) • plot from data • feed p0 spectrum into MC, simulate • get double gamma ratio Rg • mT scaling for other decay sources J. Jin - JClub

  9. Results • Ratio of direct-photon-candidates to the p0 spectrum (data & MC) • Direct g cross section • small but significant signal observed • agree with NLO pQCD J. Jin - JClub

  10. Other methods I • p0 tagging method: J. Jin - JClub

  11. Other methods II • Isolation cuts • photon energy < 10% of cone energy • cone size: 0.5 around the candidate photon • cone energy: sum of track momentum in DC & EMCal cluster energy • correct for limited PHENIX acceptance, which causes fake isolated photons • Emphasize the Compton process • direct access to gluon distribution J. Jin - JClub

  12. Results (RUN3 p+p) • direct photon production • spectrum up to 16GeV/c • well described by NLO pQCD for pT>5GeV/c • ratio of isolated photons to all direct photons • large systematics in low pT • agree with pQCD(lines) for pT>7GeV/c J. Jin - JClub

  13. Direct photons in Au+Au • What’s new in Au+Au? • centrality determination by BBC&ZDC • high pT hadron suppression in central Au+Au, measure Rg directly: • Rg > 1, indicates direct photon signal • Direct photon spectra extracted as: J. Jin - JClub

  14. Results • double gamma ratio plot • advantage: many systematics cancel • an excess over unity is observed at high pT • magnitude increases with centrality due to hadron suppression • expectation for Ncoll scaling of direct photons holds for all centrality bins J. Jin - JClub

  15. spectra shown • 9 centrality bins and MB • comparison to NLO • at low to mid-pT where additional production of thermal photons appears, we have large error bars J. Jin - JClub

  16. Improvement on mid-pT thermal photon region J. Jin - JClub

  17. goal: improve upon mid-pT region systematic errors • using most stable subset of RUN4 data • improving various corrections • p0 peak extraction • energy scale/linearity • energy smearing • fitting function to p0 spectrum • using stochastic cuts to extract photons J. Jin - JClub

  18. Going to low pT:No apparent excess J. Jin - JClub

  19. Excess above the scaled pQCD? • new points agree with, but consistently higher than the old ones ~4GeV/c J. Jin - JClub

  20. A new approach:very low-mass di-lepton pairs J. Jin - JClub

  21. g g p0 p0 e+ g* g e- The idea: Dalitz decay • mass distribution of e+e- pairs (Knoll-Wada formula): 1) pure QED part, universal to all Dalitz decays 2) phase factor part, depends on M of parent hadron 3) hadronic form factor at low mass, both the second and third part become 1 at high mass, suppressed by phase factor J. Jin - JClub

  22. e+ 0-30 90-300 MeV Rdata ÷ Compton scattering e- Now direct photons: g* q g q • Any source of real g produces virtual g with very low mass • Rate and mass distribution given by same formula • No phase space factor for mee<< pT photon We measure Rdata: N(90<Mee<300)/ N(0<Mee<30) We can calculate: Rp0, Rh, Rdirect J. Jin - JClub

  23. True in very low mass window! Rdata excess over calculated expected ratio from hadronic decays: virtual & real direct photon signal J. Jin - JClub

  24. Results: Rdata 0-20 % J. Jin - JClub

  25. g*direct/g*inclusive 0-20 % Significant 10% excess of very-low-mass virtual direct photons J. Jin - JClub

  26. more peripheral Centrality Dependence Indication for centrality dependence J. Jin - JClub

  27. ( + 1 ) Comparison to conventional results J. Jin - JClub

  28. gdirect J. Jin - JClub

  29. The Spectrum Compare to the published Run2 results (black arrow) J. Jin - JClub

  30. Compare to NLO pQCD • excess above pQCD The Spectrum J. Jin - JClub

  31. Compare to NLO pQCD • excess above pQCD Compare to thermal model • data above thermal at high pT The Spectrum J. Jin - JClub

  32. Compare to NLO pQCD • excess above pQCD Compare to thermal model • data above thermal at high pT Compare to thermal + pQCD • data consistent with thermal + pQCD The Spectrum J. Jin - JClub

  33. Backup J. Jin - JClub

  34. thermal: Decay photons hard: Schematic Photon Spectrum in Au+Au J. Jin - JClub

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