Measurement of charmonia at mid-rapidity at RHIC-PHENIX c J/ e + e - in p+p collisions at √s=200GeV Susumu Oda CNS, University of Tokyo For the PHENIX collaboration 2007/09/24 62nd annual JPS meeting Hokkaido University 1
Motivation 0 mb 3 mb Red : Au+Au |y|<0.35 Magenta : Cu+Cu |y|<0.35 Blue : Au+Au 1.2<|y|<2.2 Aqua : Cu+Cu 1.2<|y|<2.2 peripheral central 2 A paper about J/ production in Cu+Cu collisions will be submitted in few weeks. Quarkonia are good probes of QGP. J/ is the most studied quarkonium in heavy ion collisions. Feed down from c into J/ is important. Direct J/ c J/ X ’ J/ X B J/ X
Charmonium system 3 c (1S) c (2S) J/ (1S) (2S) c0 (1P) c1 (1P) c2 (1P) h c (1P) J PC e + e - BR=5.94% DDbar threshold
Fraction of J/ from c decay Measurement of c at RHIC is required. RHIC energy 4 Precise measurements only Error of R c <=0.1 CDFHERA-B E705 E672/E706 E369 WA11 E610
Theoretical model predictions Color Evaporation Model CSM+Comover NRQCDColor Singlet Model Measurement of c at RHIC is required to understand quarkonia production. 5
How to measure R c Find J/ e + e - (2.9<M ee <3.3GeV). Find c J/ ( M=M ee -M ee ~0.44GeV). Correct acceptance event by event. Subtract background by event mixing of J/ and . –Normalization regions : GeV and GeV Run-5 (2005, 3.8 pb -1 ) and Run-6 (2006, 10.7 pb -1 ) p+p 200GeV data is used. c acceptance Average over J/ 6 c conditional efficiency if J/ is detected J/ acceptance
e-e- e+e+ p p PHENIX detector Beam beam counter –Collision vertex Drift chamber, pad chamber –Charged particle tracking Ring imaging Cherenkov counter –Electron identification Electromagnetic calorimeter –Photon identification and energy measurement –Electron identification 7 p+p c +X J/ +X e + e - +X
Cut parameters and peaks Photon cut Energy cut (E >0.3 GeV) Electromagnetic shower profile Fiducial cut (noisy EMC towers are removed) Charged particle veto (35cm x 35cm) Event cut |Zvertex|<30 cm Electron cut RICH nPMT>=2 pT>0.2 GeV 0.5<Energy/momentum<2 Pair cut 2.9<mass(e + e - )<3.3GeV 8 EMCal energy resolution (E)=58MeV (PbSc) (E)=42MeV E=500MeV N J/ =3679 Red : N +- =4040 Blue : N ++ +N -- =218 J/ e + e - Run-5+6 p+p 0202 Run-6 p+p 0.6<p T <0.65GeV/c E >0.2GeV mainly 0 2 PYTHIA simulation ( c1, |y |<0.5)
J/ acceptance and c conditional efficiency if J/ is detected from GEANT (PISA) simulation 9 pT,J/ (GeV/c)pT, c (GeV/c) J/ e + e - ~2% for |y J/ |<0.5 c J/ e + e - ~10% c acceptance~2%*10%=0.2% 1/30,000 of produced c is detected by PHENIX central arm
Feasibility study using PYTHIA and GEANT simulation Black : Foreground Blue : Background Red: Foreground-background Green : Normalization regions (0.1< M<0.3GeV and 0.6< M<0.8GeV) Input R c =0.32 N(direct J/ ):N( c1 J/ ):N( c2 J/ )=68%:16%:16% N J/ =
Feasibility study using simulation (continued) 11 Input R c =0 Input R c =0.32 Input R c =0.68 Input R c =1 Black : Foreground Blue : Background Red: Foreground-background Green : Normalization regions
Real data (Run-5 and Run-6 p+p 200GeV) Black : Foreground Blue : Background Red: Foreground-background Green : Normalization regions 12 The fraction of J/ from c feed down (R c ) seems to be small.
Summary and outlook 13 RHIC energy The contribution of c is important to understand the J/ data in heavy ion collisions. Search for the c meson via J/ decay in p+p collisions is ongoing. The fraction of J/ from c feed down (R c ) seems to be small. The R c value will be obtained soon. More and more statistics are needed (Run-8, 9, …) for detail study.
Backups
Acceptance of 0
Ratio of cross sections For simplicity, I assumed Limited knowledge Green : A Blue : pA Aqua : ppbar And I used mean of masses in simulation. ( MeV MeV)/2 Expected width of the convoluted peak (Gaussian sigma) is ~50MeV. I neglect c0 contribution.
( c - J/ ) Mass (GeV/c 2 ) PHENIX Run 5 200GeV p+p ( c - J/ ) Mass (GeV/c 2 ) Previous result, Run-5 p+p 200GeV N J/ =960
Questions from audience
Question 1 Is it better to use muon pairs? The statistics of muon pairs are 5 times larger than electron pairs. No. The fraction of decays with J/ going to muon arm and gamma going to central arm is small. So, the statistics of J/ + in muon+central arms and the statistics in central arm are almost equivalent. But, the energy of gamma in muon+central arm configuration is low, E~0.1(0.2)GeV. This is worse situation than the central arm case with E~0.4GeV. Does the fact mean that the c measurement is not possible at RHIC-PHENIX? No. It can be possible in p+p collisions as I showed, while the larger statistics is needed. But, it is very hard in heavy ion collisions. (The measurement with NCC+FVTX+muon trigger is interesting even in heavy ion collisions, but the statistics are still necessary.)
Question 2 Is it better to use the isolation cut ( 0 veto)? No. The acceptance of low-pT 0 is small, ~3% at 0 p T =0.6GeV/c (slide 15). So, the 0 veto will not be effective for the c measurement and it will introduce larger systematic error only.
Question 3 You did not take into account the c0 contribution in your PYTHIA+GEANT simulation. Why? Because the branching ratio of c0 J/ decay is small, only 1.3% (slide 3). If the production cross section of c0 is much larger than ones of c1 and c2, we cannot neglect it. But, we know the c0 fraction is not large in the fixed target experiment and Tevatron. So, the c0 contribution can be negligible.
Question 4 In the PYTHIA+GEANT simulation, the net (foreground- background) distribution is lower than zero at the lower side of the c peak. Why? There is correlation between J/ and in the foreground and most of come from 0 2 decays. The correlation leads high mass of J/ pairs. In the background, however, there is no correlation between J/ and , and the J/ pair mass is smaller. Therefore, the net distribution is lower than zero in simulation in the lower side. But, actually, this fact is not observed in the real data. Though, the J/ statistics is the same in the simulation and real data. Why don’t you include such effect in your background subtraction? I don’t believe the prediction capability of PYTHIA at such level.