1 / 34

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

Download Presentation

PHENIX measurement on direct photon production

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related