1. 03/13/2012Moriond QCD and High Energy Interactions, Mar. 10-171 Di-lepton production at STAR Outline: Motivation and Introduction Recent results from STAR Future perspectives from STAR Conclusions and outlook Lijuan Ruan for the STAR Collaboration (Brookhaven National Laboratory)

2. 03/13/2012Moriond QCD and High Energy Interactions, Mar. 10-172 S. Bass Probe the medium properties H. van Hees and R. Rapp, Nucl.Phys.A806 (2008) 339; T. Renk and J. Ruppert, PRC77(2008)024907 In medium modification of vector mesons and thermal radiation: di-lepton continuum NA60, PRL100,022302(2008) NA60, Eur.Phys.J.C59,607(2009)

3. 03/13/2012Moriond QCD and High Energy Interactions, Mar. 10-173 STAR detector: TPC & MRPC-TOF A recent technology (TOF) ---- Multi-gap Resistive Plate Chamber adopted from CERN-ALICE. 1.Good timing resolution, ( , K) < 1.6 GeV/c, proton < 3 GeV/c 2.Coverage: -0.9<  <0.9 3.Coverage (4% y2008, 72% y2009, 100% y2010) Time Projection Chamber 1.Tracking 2.Ionization energy loss (dE/dx): ( , K) 3 GeV/c, proton 3 GeV/c 3.Coverage -1<  <1

4. 03/13/20124 STAR Time of Flight detector performance STAR, PLB616(2005)8 |1/  -1|<0.03 STAR, PRL94(2005)062301 TOF PID: ( , K) ~ 1.6, proton ~ 3 GeV/c STAR Collaboration, PLB616(2005)8 TOF enables clean electron PID up to P T < 3 GeV/c. STAR Collaboration, PRL94(2005)062301 M. Shao et al., NIMA 558(2006)419 Moriond QCD and High Energy Interactions, Mar. 10-17 200 GeV d+Au collisions

5. Di-electron signal 03/13/20125Moriond QCD and High Energy Interactions, Mar. 10-17 Di-electon signal: e+e- pairs from light flavor meson and heavy flavor decays (charmonia and open charm correlation): Pseudoscalar meson Dalitz decay:  0, η, η'   e + e - Vector meson decays:  0, ,   e + e -,    0 e + e -,   ηe + e - Heavy flavor decays: J/   e + e -, ccbar  e + e - X, bbbar  e + e - Drell-Yan contribution In Au+Au collisions, we search for QGP thermal radiation at 1.1<M ee <3.0 GeV/c 2 (intermediate mass range) Vector meson in-medium modifications at M ee <1.1 GeV/c 2 (low mass range)

6. Di-lepton production in 200 GeV p+p 03/13/20126Moriond QCD and High Energy Interactions, Mar. 10-17 STAR, QM2011,SQM2011 cocktail simulation is consistent with di-electron spectrum in p+p collisions at 200 GeV charm correlation contribution dominates in the intermediate mass region (1.1-2.9 GeV/c 2 ) Simulation: charm correlation contribution is from PYTHIA. STAR acceptance: |y ee | 0.2 GeV/c    J/ 

7. Di-lepton spectra in 200 GeV Au+Au 03/13/20127Moriond QCD and High Energy Interactions, Mar. 10-17 STAR, QM2011, SQM2011 Minbias (value ± stat ± sys)Central (value ± stat ± sys) STAR 1.53 ± 0.07 ± 0.41 (w/o ρ) 1.40 ± 0.06 ± 0.38 (w/ ρ) 1.72 ± 0.10 ± 0.50 (w/o ρ) 1.54 ± 0.09 ± 0.45 (w/ ρ) possible enhancement factor in 0.15<M ee <0.75 Gev/c 2 PHENIX results, see K. Okada’s talk. J/        0-80% Au+Au 0-10% Au+Au

8. 03/13/20128Moriond QCD and High Energy Interactions, Mar. 10-17   ee spectra in 200 GeV p+p and Au+Au  ω  ee measurements via di-lepton channel in p+p and Au+Au at STAR  ω  ee flow pattern is similar to light hadrons Tsallis Blast-wave(TBW) fit: =0 in p+p, =0.47 in 0-80% AuAu. Z.Tang et al., arXiv:1101.1912 STAR, QM2011, SQM2011 STAR Preliminary

9. 03/13/20129Moriond QCD and High Energy Interactions, Mar. 10-17   ee spectra in 200 GeV Au+Au  The invariant yield of  via di-lepton channel in Au+Au is consistent with that from hadronic decay channel.  mass and width between data and simulation: consistent STAR, SQM2011 STAR Preliminary

10. Di-electron v 2 versus p T & M ee : probe the properties of the medium from hadron-gas dominated to QGP dominated constrain QGP dynamics (η/s, T, t 0 …) v 2 versus M ee at 200 GeV Au+Au 03/13/201210Moriond QCD and High Energy Interactions, Mar. 10-17 A factor of 2 more minbias data from run11 is on disk. STAR, SQM2011, DNP2011 STAR Preliminary

11. 11 To provide a comparison (blue) curve from simulation: 1. Parameterize  meson v 2 results 2. Do the Dalitz decay simulation and obtain expected v 2 of di-electrons pairs from  0 Dalitz decay. 3. This is consistent with our di-electrons v 2 results. To provide a comparison (blue) curve from simulation: Assume η v 2 same as K S do the same Dalitz decay procedure v 2 of di-electrons from π 0 and η Dalitz decay STAR Preliminary PRC 80, 054907 (2009) 03/13/2012Moriond QCD and High Energy Interactions, Mar. 10-17 STAR, SQM2011, DNP2011

12. Measure charm correlation with MTD upgrade: ccbar  e+  03/13/201212Moriond QCD and High Energy Interactions, Mar. 10-17 e  correlation simulation with Muon Telescope Detector (MTD) at STAR from ccbar: S/B=2 (M eu >3 GeV/c 2 and p T (e  )<2 GeV/c) S/B=8 with electron pairing and tof association MTD: construction starts in FY2011; project completion in FY2014 Z. Xu, BNL LDRD 07-007; L. Ruan et al., JPG36 (2009) 095001 R. Rapp, hep-ph/0010101

13. 03/13/2012Moriond QCD and High Energy Interactions, Mar. 10-1713 Di-lepton measurements at STAR FY09/10/11FY12/13From FY14 on Detector components TPC+TOF+EMCTPC+TOF+EMC+MTD(p)TPC+TOF+EMC+MTD+ HFT Measurements J/   ee, Upsilon  ee J/  R AA, v 2 vs. p T &N part, Upsilon R AA vs. N part di-electron continuum di-electron spectra, v 2 vs. p T FY12 (10% MTD): first e-µ measurement FY13 (43% MTD): J/   µµ, Upsilon  µµ J/  R AA, v 2 vs. p T & N part, first look at different Upsilon states di-muon continuum e-µ v 2 B  J/  X  µµX different Upsilon states R AA versus N part, J/   µµ R AA, v 2 versus p T & N part, di-muon continuum, e-µ spectra and v 2 D v 2 and R AA PhysicsColor screening features, quarkonia production mechanisms, vector meson in-medium modifications, low mass enhancement, Intermediate mass Better understanding of thermal radiation from QGP at intermediate mass and vector meson in- medium modifications Measure thermal radiation from QGP at intermediate mass, vector meson in- medium modifications, color screening features

14. 03/13/2012Moriond QCD and High Energy Interactions, Mar. 10-1714 Summary STAR has started the di-lepton program with a bright future enabled by the TOF upgrade : Di-lepton spectrum in 200 GeV p+p collisions: Cocktail simulation consistent with data Charm correlation contribution dominates at intermediate mass Di-lepton in 200 GeV Au+Au collisions: A possible low mass enhancement with respect to cocktail expectation at M ee 0.15-0.75 GeV/c 2   ee shows a similar flow velocity at freeze-out as light hadrons  yields are similar between di-leptonic decay and hadronic decay channel Di-lepton v 2 from low to high mass measured Towards the future: Differential measurements (M ee, p T, v 2 ) are on-going Energy dependence (19-200 GeV) can be systematically studied at STAR e-µ correlation (spectrum and v 2 ) to distinguish heavy flavor production from initial lepton pair production with MTD upgrade

15. 03/13/2012Moriond QCD and High Energy Interactions, Mar. 10-1715 Backup

16. Compared to theoretical calculations within STAR acceptance Blue dotted: HG_medium Pink dotted: QGP Solid lines: upper: cocktail + HG+QGP lower: cocktail HG_med, QGP from R. Rapp (private communication) R. Rapp and J.Wambach, Adv. Nucl. Phys. 25, 1 (2000). 03/13/2012Moriond QCD and High Energy Interactions, Mar. 10-1716

17. The elliptic flow of   ee is consistent with the result of   KK within errors STAR preliminary   KK: PRL99(2007)112301 v 2 of di-electrons at 0.5<M ee <1.1 GeV/c 2 STAR, DNP2011 03/13/201217Moriond QCD and High Energy Interactions, Mar. 10-17

18. 03/13/2012Moriond QCD and High Energy Interactions, Mar. 10-1718 The most recent quarkonium results 1.J/ψ R AA increase from low to high p T 2.Not suppressed in peripheral Au+Au but suppressed in central Au+Au at high p T 3.No significant v 2 4.Upsilon suppressed in Au+Au STAR, QM2011, SQM2011

19. 03/13/2012Moriond QCD and High Energy Interactions, Mar. 10-1719 Data sets for di-electron analyses Run periodCollision energyBeam speciesDetector 2009200 GeVp+pTPC+TOF(72%) +EMC 2010200 GeVAu+AuTPC+TOF+EMC 62 GeVAu+AuTPC+TOF+EMC 39 GeVAu+AuTPC+TOF+EMC 2011200 GeVAu+AuTPC+TOF+EMC 27 GeVAu+AuTPC+TOF+EMC 19 GeVAu+AuTPC+TOF+EMC large acceptance of the TOF system and low material budget * at mid-rapidity * beam pipe (0.29% radiation length), beam pipe wrap (0.14%), air (0.17%), IFC (0.45%)

20. Electron identification capability 03/13/201220Moriond QCD and High Energy Interactions, Mar. 10-17 |1/β-1|<0.03|1/β-1/β expected |<0.025 Clean electron identification obtained with TPC+TOF: Electron purity: 99% in p+p and 97% in minbias Au+Au Hadron contamination contribution to di-lepton is insignificant and taken as part of systematic uncertainties. 200 GeV p+p collisions

21. Di-electron signals in p+p and Au+Au STAR preliminary p+p @ 200 GeV 03/13/201221Moriond QCD and High Energy Interactions, Mar. 10-17

22. Di-lepton background Mixed-event normalized to like-sign within 0.4 - 1.5 GeV/c 2 Cross-pair bg. can only be described by like-sign method. For minimum bias events, we subtracted like-sign bg at M ee 0.7 GeV/c 2. 03/13/201222Moriond QCD and High Energy Interactions, Mar. 10-17

23. Di-lepton azimuthal angle distribution Mixed-event normalized to like-sign within 0.4 - 1.5 GeV/c 2 Cross-pair bg. can only be described by like-sign method. For minimum bias events, we subtracted like-sign bg at M ee 0.7 GeV/c 2. 03/13/201223Moriond QCD and High Energy Interactions, Mar. 10-17 STAR preliminary

24. Di-lepton background Mixed-event normalized to like-sign within 1 - 3 GeV/c 2 Cross-pair bg. can only be described by like-sign method. For minimum bias events, we subtracted like-sign bg at M ee 0.7 GeV/c 2. 03/13/201224Moriond QCD and High Energy Interactions, Mar. 10-17

25. Low mass enhancement PHENIX PRC 81 (2010) 034911 Minbias (value ± stat ± sys)Central (value ± stat ± sys) STAR 1.53 ± 0.07 ± 0.41 (w/o ρ) 1.40 ± 0.06 ± 0.38 (w/ ρ) 1.72 ± 0.10 ± 0.50 (w/o ρ) 1.54 ± 0.09 ± 0.45 (w/ ρ) PHENIX 4.7 ± 0.4 ± 1.57.6 ± 0.5 ± 1.3 Difference2.0 σ4.2 σ Enhancement factor in 0.15<M ee <0.75 Gev/c2 Note: Acceptance difference etc. 03/13/201225Moriond QCD and High Energy Interactions, Mar. 10-17

26. Systematic uncertainties p+p:Au+Au: STAR preliminary 03/13/201226Moriond QCD and High Energy Interactions, Mar. 10-17

27. Reproduce PHENIX cocktail Reproduce the cocktail within PHENIX acceptance by our method. The momentum resolution are still from STAR. Scaled by all the yields from PHENIX paper[1], we can reproduce the PHENIX cocktail. [1]. Phys. Rev. C 81, 034911 (2010). STAR preliminary 03/13/201227Moriond QCD and High Energy Interactions, Mar. 10-17

28. Check with acceptance difference Scaled by same meson and charm yields. Scaled by the acceptance difference Difference at low mass is not from the simulation but from the measurements. Acceptance difference: STAR preliminary 03/13/201228Moriond QCD and High Energy Interactions, Mar. 10-17

29. v 2 standard event-plane method ν 2 Total is flow of unlike-sign pairs. v 2 B (M) is flow of background calculated using the like- sign or mixed events pairs. v 2 S is flow of signal. N S is the signal number, N B is the background (like-sign) number. N (S+B) is unlike-sign number. 03/13/201229Moriond QCD and High Energy Interactions, Mar. 10-17

30. Unlike-sign and background v 2 Unlike-sign v 2 Background v 2 Signal/(Signal+Background) Signal flow STAR preliminary 03/13/201230Moriond QCD and High Energy Interactions, Mar. 10-17