1. Low-mass dielectron measurement at RHIC-PHENIX Yoshihide Nakamiya (for the PHENIX collaboration) Hiroshima University, Japan 1

2. 2 Outline  Physics goals for low-mass dielectron measurement  Difficulties and Challenges for di-electron analysis in Au+Au collisions  Low-mass vector mesons (  ) at low pT Mass shape Yield comparison among several decay modes.  Low-mass continuum at low pT. Continuum yield  Summary

3. 3 Physics of low-mass dielectron R. Rapp J. Phys G31 (2005) S217 M.Harada et. al. Phys.Rev.D74 :114006,2006. In hadronic phase In partonic phase RHIC  High energy heavy-ion collisions produced extremely high-temperature medium.  Several kinds of model calculations predict significant change of spectral function (but tendency of the change is different between them)  My motivation is what behavior is experimentally observed ? KEK-PS/E325

4. Low-mass vector mesons as a probe  Mass spectrum shape Careful investigation of mass shape of light vector mesons are essential to discuss chiral symmetry restoration. It is important to measure transparent probes like  e + e - decay modes because electrons can pass through the medium with few interactions and therefore carry original information on the meson production.  Yield difference between  e + e - and  K + K - Branching ratio between e + e - and K + K - may be sensitive to mass modification, since M phi is approximately 2  M K. Need to compare yield of  e + e - and  K + K - T.Hatsuda and S.Lee Phys.Rev.C46-1 1992 m KK mm 4

5.  CERES/NA45 (past experiment ) Enhancement of di-electron continuum had been discovered at CERES/NA45. Some theoretical comparisons had been done. ex)  dropping, collision broadening …  PHENIX experiments Detailed analysis become possible at PHENIX since continuum distribution can be separated from mass peak of light vector mesons. Low-mass dielectron continuum as a probe Rapp-Wambach, 2000 ---  dropping --- collision broadening 5

6.  Systematic studies give much information about properties of hot medium. several collision species - p+p (as baseline) - d+Au (examine cold nuclear effects) - Au+Au (examine effects in partonic matter) several decay channels  All data shown in this presentation are taken in 200 GeV (were taken in run3 - run5) Fundamental strategy for low-mass dielectron measurement at PHENIX   mesons  -> e + e - BR = 10 -4  -> K + K - BR = 50%   mesons  -> e + e - BR = 10 -5   ->  0  BR = 9%  ->  0  +  - BR = 90% 6

7. Particle detection at PHENIX e+e+  ee  Acceptance -0.35 < η < 0.35 2 x 90°in azimuthal angle for two arms  Electron detection precision tracking : DC/PC electron id : RICH, EMCal etc  e separation is 1/10 -4 at pT < 4.7 GeV/c The PHENIX spectrometers are excellent devices to measure electrons. (can also measure hadron/photon at the same time, it make systematic measurement possible) 7

8. Uncorrelated combinatorial pair Difficulty(1) High multiplicity dN ch /d  ~ 650 ( in most central Au+Au collision at mid-rapidity) Difficulty(2)Huge combinatorial background Dalitz decay (    e + e - ) photon conversion Difficulty(3) Small fraction to dielectron decay mode from  Branching Ratio (BR )  -> e + e - BR = 10 -4  -> K + K - BR = 50%  -> e + e - BR = 10 -5  ->  0  BR = 9%  ->  0  +  - BR = 90% 0  PRL 91 052303(2003) 5-5  0  e + e -  e + e - Difficulties in di-electron analysis in Au+Au 200GeV ex) It is a key to overcome their difficulties by analysis efforts. 8

9. Key challenges for di-electron analysis in Au+Au 200GeV Difficulty(1) High multiplicity accidental charged hadron contamination unphysical correlated background Difficulty(2) Huge combinatorial background background by uncorrelated pairs Difficulty(3) Small fraction to dielectron decay mode not avoidable (decided by physics) 9 Challenge against each item are shown from the next slide.

10. Number of PMT distribution in RICH is different between real data and simulation due to random association tracks.  electron ID by RICH Charged hadron tracks are detected as electrons by random associations under high multiplicity environment. Black : electron identified tracks (real data) Red : electrons (simulation) Electron identification in high multiplicity environment (Au+Au 200GeV) 10

11. Black : electron identified tracks (real data) Blue : random associated tracks (real data) Detector response under high multiplicity environment is reasonably understood and evaluated. Black : electron identified tracks after subtracting random associated tracks Red : electrons (simulation)  electron ID by RICH Charged hadron tracks are detected as electrons by random associations under high multiplicity environment. 11 Electron identification in high multiplicity environment (Au+Au 200GeV)

12. Blue : opening angle between two tracks on RICH plane(same event) Red : opening angle between two tracks on RICH plane(event mixing) Effect from unphysical correlated tracks are understood and evaluated. RICH plane Unphysical correlated background in high multiplicity environment (Au+Au 200GeV)  unphysical correlated background in RICH When a  points to the same ring as an electron, it is associated to the same ring. This happens for a typical values of opening angle which folded with the average momentum of the electron corresponded to a particular invariant mass. 12

13. Multiplicity effects to E/p are reasonably evaluated and calibrated.  hadron rejection by Energy-momentum matching Energy-Momentum matching (E/P) is distributed about 1. small electron mass compared to measured momentum scale hadrons deposit only MIP energy in EMCal. Black : electron identified tracks (real data) Red : random associated tracks (real data) Black : electron identified tracks after subtracting random associated tracks 13 Electron identification in high multiplicity environment (Au+Au 200GeV)

14. Combinatorial background (Au+Au 200GeV) --- same event N +- --- mixed event N +-  shape from mixed event Good agreement for uncorrelated distribution in same event  normalization between same event and mixed event Normalize B ++ and B  to N ++ and N . Normalize mixed +- pairs to  →ee  J   14 Combinatorial background from uncorrelated pairs are evaluated.

15. 15 Results of low-mass vector mesons

16. 16 Result of mass shape analysis for  e + e - in Au+Au 200 GeV  It is successful to extract  signals in Au+Au 200 GeV … so far we cannot discussed  mass shape due to lack of statistics and signal to background ratio. need more statistics (3 times as many statistics as run4 has already taken at run7 ) need to improve S/N (upgrade devices )  

17. Future improvement of dielectron measurement by Hadron Blind detecor (HBD )  Remaining problems We made analysis effort and extract the signal in Au+Au 200 GeV. But the following cases cannot be improved by analysis. case1) Counter part of electron pair go out from the PHENIX acceptance case2) Very low pT electrons cannot escape the magnetic field  Solutions HBD detector has capability to identify Dalitz decaying electrons via their opening angle. S/B will be expected to improve a factor 10 to 100. 17 e+e+ e-e- 0e+e-0e+e- Case 1 e+e+ e-e- Case 2 0e+e-0e+e- HBD

18. Extended  K+K- analysis  The three independent kaon analyses (double kaon id, no kaon ID and single kaon ID methods) are consistent with each other.  Spectra between K + K - and e + e - are reasonable agreement in p+p. d+Au 0-20%  p+p No ID Single ID Double ID M.B. Consistency check for  mesons in p+p/d+Au 200 GeV d+Au 0-20%  p+p No ID Single ID Double ID e + e - M.B. 18

19. Consistency check for  mesons in p+p/d+Au 200GeV   spectra also show good agreement among several decay channels. p+p d+Au   0  dAu MB (PRC75 151902)  0  +  - dAu MB(PRC75 151902)  e + e - pp MB (PHENIX preliminary)  0  pp MB (PRC75 151902)  0  +  - pp MB(PRC75 151902)  0  pp ERT (PHENIX preliminary)  0  +  - pp ERT (PHENIX preliminary) 19

20.  e + e - AuAu MB  e + e - 20-40% x 10 -3  e + e - 40-92% x 10 -1  K+K- AuAu MB (no PID)  K+K- AuAu MB (double PID)  K+K- AuAu MB (PRC72 014903)  K+K- 20-40% x 10 -3 (double PID)  K+K- 40-92% x 10 -1 (double PID)  K+K- 40-92% x 10 -1 (PRC72 014903)  It’s successful to measure p T spectra of  e + e - in Au+Au. … so far we cannot make any clear statement about the comparison between spectra for  e + e - and      More statistics and improvements of S/N are needed. 40-92% 20-40% Au+Au p T spectra comparison for  mesons in Au+Au 200 GeV M.B.   K + K - 20

21. p T spectra for  mesons in Au+Au 200GeV   measurement are done up to 10 GeV/c.  Long awaited   e  e , along with   e  e  0-20% 20-60% 60-92% M.B. 21

22. 22 Results of low-mass dielectron continuum

23. Hadronic source of e  e   “ Cocktail” of e  e  from hadron decays Dalitz decays  0  e  e ,  0 e  e  Direct decays  e  e   J    e  e  Semileptonic decays of heavy flavor Drell Yan process  Estimate contribution from hadron decay Fit  0  and  ± data For ,J/  mesons, measured yields are used(in Au+Au/p+p) For ther hadrons, we use normalization A under assumption they follows “m T scaling”  Charm production  c = N coll x 567±57±193  b from single electron measurement. Binary scaling (N coll ) to Au+Au. 23

24. Continuum Extraction in p+p 200 GeV  Signal/Background ~ 1.  Peaks have well separated due to high resolution.  The main components of backgrounds are taken into account. Uncorrelated background by event mixing Contamination from “jet pair” and “cross pair” is taken into account. π0π0 π0π0 e+e+ e-e- e+e+ e-e- γ γ π0π0 e-e- γ e+e+ “Jet pairs” “Cross pairs” X Correlated Signal = Data-Mix Mixed events “Jet pairs” “Cross pair” 24

25. Low-mass dielectron continuum in p+p 200 GeV  Excellent agreement has seen between data and hadron decay contribution from “cocktail” analysis.  Charm cross section ( integration after cocktail subtraction)  c =544 ± 39 (stat) ± 142 (sys) ± 200 (model)  b 25

26. Continuum Extraction in Au+Au 200 GeV  Signal/Background ~ 1/200.  Peaks have well separated due to high resolution  Signal is also extracted in Au+Au 200 GeV. Mixed events Correlated Signal = Data-Mix 26

27. Low-mass dielectron continuum in Au+Au 200 GeV Charm from PYTHIA filtered by acceptance  c = N coll x 567±57±193  b Charm “thermalized” filtered by acceptance  c = N coll x 567±57±193  b  Continuum enhancement is observed in 150 < m ee <750 MeV.  Intermediate-mass dielectron continuum are consistent with PYTHIA if charm production from thermal radiation is taken into account. 27

28. 0<p T <0.7 GeV/c 0.7<p T <1.5 GeV/c 1.5<p T <8 GeV/c 0<p T <8.0 GeV/c p+p Au+Au arXiv: 0802.0050 arXiv: 0706.3034  Low mass excess in Au+Au is concentrated in low pT. (Medium dependent effects) p T dependence of continuum enhancement 28 0 < p T < 8.0 GeV/c0 < p T < 0.7 GeV/c 0.7 < p T < 1.5 GeV/c 1.5 < p T < 8.0 GeV/c

29. Centrality dependence of continuum enhancement  Mass window (a) 150 < m ee < 750 MeV (b) 0 < m ee < 100 MeV   0 region (m ee <100 MeV) production scales approximately with N part  Excess region (150<m ee <750 MeV) production from hot matter.  scattering processes like  or qq annihilation) a yield rising faster than proportional to N part 29

30. Mass dependent dielectron p T spectra p+p Au+Au  p+p data are consistent with “cocktail” up tp 3 GeV/c  Au+Au data enhanced for all p T at the range above m  30

31. Comparison to theoretical model (in Au+Au)  Both  dropping and collision broadening models had been well reproduced in CEREC/NA45.  Any models are not described continuum enhancement in PHENIX !?  Theoretical models vacuum spectral function  dropping collision broadening Rapp-Wambach, 2000 CERES/NA45 ---  dropping --- collision broadening PHENIX 31

32.  Di-electron analysis Difficulties of dielectron analysis in Au+Au 200 GeV are identified, attacked and overcome.  Low-mass vector meson Make sure that p T spectra between several decay channels are consistent in p+p. It is successful to extract  signals in Au+Au 200 GeV. (but statistical and systematic errors are large so far) 3 times as many statistics as at run4 has already taken at run7 signal/background will be improved by HBD.  Low-mass dilelectron continuum Dielectron mass spectra are well reproduced by “cocktail” analysis in p+p 200GeV. Enhancement of continuum yields is observed in 150 < M ee < 750 MeV/c 2 in Au+Au 200GeV. Effective theoretical models to explain the enhancement are needed. 32 Summary

33. 33 I’m glad to join in Nagoya workshop ! I’d like to say thanks to all participants !

34. Back up 34

35. 35 Generated electrons in Au+Au 200GeV

36. 36 After several experimental checking, we found integrated information is not enough to determine the difference between e + e - and K + K -. Need a careful check of m T spectra. Yield and temperature for  mesons

37. 37 The invariant mass spectra for φ mesons  e + e - p+p  e + e - d+Au  e + e - Au+Au φ φ  K + K - (double PID) d+Au  K + K - (double PID) Au+Au

38. 38 The invariant mass spectra for φ mesons      no PID) AuAU      single PID) pp      no PID) pp      no PID) dAu

39. 39 The invariant mass spectra for ω mesons  e + e - p+p  e + e - d+Au  e + e - Au+Au ω ω       p+p  0  p+p  0  Au+Au