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Measurement of high momentum photons at RHIC-PHENIX

Measurement of high momentum photons at RHIC-PHENIX. Kensuke Okada RIKEN BNL Research Center for the PHENIX collaboration JPS meeting (Sep.22, 2007). High momentum  is important. In the direct photon analysis, the basic strategy is

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Measurement of high momentum photons at RHIC-PHENIX

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  1. Measurement of high momentum photons at RHIC-PHENIX Kensuke Okada RIKEN BNL Research Center for the PHENIX collaboration JPS meeting (Sep.22, 2007) K.Okada (RBRC)

  2. High momentum  is important In the direct photon analysis, the basic strategy is (direct photon) = (all photon) – (background photons) BG photons are mostly hadronic decays. These points are important K.Okada (RBRC)

  3. PHENIX EMCal Mid rapidity, 5m radial from beam pipe, +/-2m along z g 2 types PbSc (6sectors) PbGl (2 sectors) K.Okada (RBRC)

  4. How photon clusters look like?  ~10GeV clusters (PbSc) data 0 2gamma distance > 27 tower / E0 [GeV] (PbSc) 20GeV pi0: 1.4tower separation of 2*10GeV clusters K.Okada (RBRC)

  5. High pT photons A concern two photons from pi0 merges They confuse the pi0 measurement for the BG estimation. Merged clusters look like a single photon. There are 3 approaches to study this issue in PHENIX • (MC) Photon shape parameterization with test beam + GEANT • (MC) Full GEANT simulation • (DATA) Real data The newest Run6pp data set is useful p+p collisions: it’s clean compared to HI collisions Run6: the highest statistics of p+p data (L=10pb-1) In this talk, I will show a data analysis of pi0 photon merge. K.Okada (RBRC)

  6. Analysis procedure Data: Run6 p+p 4pb-1 (out of 10pb-1) There are non-collision related clusters (e.g. cosmic-ray, beam background) 1  They are corrected by using EMCal ToF information. 2 4 categories Shape check (1 photon assumption) Associated track check (DC match)  = A: photon shape, no DC B: not photon shape, no DC C: photon shape, with DC D: not photon shape, with DC • Real photons • Neutral hadrons|| ~2% of photons || merged pi0 • Charged hadron || electron • Charged hadron The target momentum region is where 2 photons from pi0 merge but not completely. K.Okada (RBRC)

  7. EMCal ToF (plots from PbSc) Black: all Red: bad shape (prob<0.02) e[GeV] PbSc Sigma~0.8 [ns] Signal : |ToF|<2 [ns] BG : 5<|ToF|<15 [ns] PbGl Sigma~1.1 [ns] Signal : |ToF|<4 [ns] BG : 6<|ToF|<22 [ns]

  8. PbSc (west arm) A: photon shape, no DC B: not photon shape, no DC C: photon shape, with DC D: not photon shape, with DC 4 categories B A In ToF Out ToF C D E [GeV] In E<~5GeV, offline threshold effect. Most of non collision related background is in the category-B.

  9. --Cluster Contents-- (After out-ToF subtraction) A B C D Yield Includes merged pi0 5~9GeV neutral hadron 10~ +merged pi0 PbSc Energy[GeV] After out-ToF subtraction A B C D 5-10 green > blue 12~ merged pi0? PbGl

  10. Merged pi0, separated pi0 extraction Merged pi0 0_merge = (not photon, no DC) – (photon, no DC)*(1-)/ - (neutral hadron) • : prob cut efficiency (measured by pure photons = under pi0 peak, good partner) PbGl PbSc (Neutral hadron): DC match cluster is scaled to give no merged pi0 below 9GeV. Separated pi0 (for the comparison) 0_sep = identified by the invariant mass of two photons

  11. 0_sep and 0_merge pi0_sep (with A<80%) pi0_sep+merged Ratio= Pi0_sep Pi0_merge PbSc PbGl is to be checked

  12. Comparison pi0_sep (with A<80%) pi0_sep+merged Ratio= MC1: photon shape parameterization MC2: GEANT (PbSc only) Data: This result PbGl MC1 MC2 Data PbSc The agreement is very nice! Are there some indication in high E region?

  13. Summary The MC result of 0 merging effect has been confirmed with real data. Assumptions: neutral hadron contribution is proportional to charged hadron contribution no pi0 merge component in E0<9GeV all merged pi0’s don’t pass the single photon criteria With more statistics, it is possible to define the merging probability with data. The effect of merged photons passing the single photon shape criteria will appear at that stage. K.Okada (RBRC)

  14. Merged pi0 yield Red: PbSc-w Blue: PbGl (PbSc) merge=(bad shape,noDC) -0.02* photon -0.385*DC (PbGl) merge=(bad shape,noDC) -0.02* photon -0.13*DC There is a clear signal in PbSc. There are some in PbGl.

  15. PbGl A: photon shape, no DC B: not photon shape, no DC C: photon shape, with DC D: not photon shape, with DC B A In ToF Out ToF C D E [GeV]

  16. EMCal ToF (PbGl) The resolution is worse, since there is no additional tuning for the walk. e[GeV] Black: all Red: bad shape (prob<0.02) Sigma~1.1 [ns] Signal : |ToF|<4 [ns] BG : 6<|ToF|<22 [ns]

  17. Cluster contents (PbSc-w) After out-ToF subtraction 5~9GeV neutral hadron 10~ +merged pi0

  18. Cluster contents (PbGl) After out-ToF subtraction 5-10 green > blue 12~ merged pi0?

  19. Results pi0_sep (with A<80%) pi0_sep+merged Ratio= AN490, Fig.13 Sasha’s new curve (private communication) Run6 4pb-1 (~50%) Use ToF information to estimate backgrounds Assumption: All merged cluster don’t satisfy the photon shape requirement. (ignore the (2) effect in the previous slide) Bad shape clusters below 9GeV are from hadronic interactions which are proportional to the charged hadron contribution. The agreement is very nice! Details https://www.phenix.bnl.gov/WWW/p/draft/okada/070321/spinpwg_20070321.pdf https://www.phenix.bnl.gov/WWW/p/draft/okada/070720/okada_feststatus_20070720.pdf

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