1. 4/1/2010MRPC workshop at USTC, Lijuan Ruan (BNL)1 STAR di-lepton measurements Outline: Motivation and Introduction Results from 200 GeV p+p collisions Projections from 200 GeV Au+Au collisions Conclusions and outlook Lijuan Ruan for the STAR Collaboration (Brookhaven National Laboratory)

2. 4/1/2010MRPC workshop at USTC, Lijuan Ruan (BNL)2 What have we learnt so far at RHIC A hot, dense medium with partonic degrees of freedom created at RHIC: Jet quenching Baryon enhancement, number of constituent quark scaling in elliptic flow … Next: Is the system thermalized and how does the system thermalize? What are the properties of the strongly-coupled system? What is the phase structure of QCD matter? What exotic particles are produced at RHIC? What is the mechanism for partonic energy loss? Does QCD matter demonstrate novel symmetry properties? What is the nature of the initial state in nuclear collisions …

3. 4/1/2010MRPC workshop at USTC, Lijuan Ruan (BNL)3 STAR’s di-lepton program The initial temperature of sQGP; the mass origin of hadrons; color screening features of heavy quarkonia … MeasurementsPhysics low mass di-leptonsthermal radiation of QGP; in-medium modifications of vector meson (    ), chiral symmetry restoration intermediate mass di-leptonsthermal radiation of QGP; heavy flavor modification; resonances in sQGP large mass: heavy quarkoniaT of QGP, color screening, quarkonium production mechanism

4. 4/1/2010MRPC workshop at USTC, Lijuan Ruan (BNL)4 Di-electron continuum at NA60 NA60, PRL96,162302(2006) 1.In-medium modifications of rho0 spectra in In-In collisions at NA60 2.Intermediate mass di-electrons show less radial flow. NA60, PRL100,022302(2008)

5. 4/1/2010MRPC workshop at USTC, Lijuan Ruan (BNL)5 Di-lepton Spectrum at PHENIX PHENIX, PRC81,034911(2010) 1.In-medium modifications of rho0 can not explain the enhancement at 0.15-0.7 GeV/c 2 2.Charm contribution to di-lepton spectra is significant at low mass at RHIC. 3.Charm contribution to di-lepton spectra might be dominant in the intermediate mass region. Its correlation makes a big difference to access the thermal radiation contribution from QGP.

6. 4/1/2010MRPC workshop at USTC, Lijuan Ruan (BNL)6 STAR detector: TPC, EMC & MRPC-TOF A new 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 -1.8<  <1.8 Electro-magnetic Calorimeter 1.electrons and photons

7. 4/1/20107 STAR Time of Flight detector performance STAR Collaboration, nucl-ex/0309012 |1/  -1|<0.03 STAR Collaboration, nucl-ex/0407006 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 MRPC workshop at USTC, Lijuan Ruan (BNL) 200 GeV d+Au collisions

8. 4/1/20108 Electron ID in p+p: high efficiency and purity high electron purity: 99% Efficiency: greater than 60% on the dE/dx cut Acceptance for |eta|<1 with TOF in y2009: 56% In y2010: 78% MRPC workshop at USTC, Lijuan Ruan (BNL) STAR Preliminary The dE/dx cut efficiency on e STAR Preliminary f(p T )

9. 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. 4/1/20109MRPC workshop at USTC, Lijuan Ruan (BNL)

10. 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. 4/1/201010MRPC workshop at USTC, Lijuan Ruan (BNL)

11. Di-lepton invariant mass spectrum in p+p collisions --- low mass range di-lepton at y2009 p+p collisions: signal excess above background ω: S/B~0.7, significance: 5 σ  : S/B~1, significance: 4 σ 4/1/201011MRPC workshop at USTC, Lijuan Ruan (BNL)

12. Di-lepton invariant mass spectrum in p+p collisions --- full range S/B ranges from 0.1 to 100 versus mass in p+p collisions 4/1/201012MRPC workshop at USTC, Lijuan Ruan (BNL)

13. 4/1/2010MRPC workshop at USTC, Lijuan Ruan (BNL)13 Cocktail Simulation in 200 GeV p+p at STAR Particle spectra from data were fit with Tsallis function, and were used as an input to full GSTAR simulation to generate e+e- invariant mass cocktail. Same cuts were applied in simulation as in data. Z. Tang et al., PRC 79,051901(R)(2009); M. Shao et al., J. Phys. G: Nucl. Part. Phys. 37 (2010) 085104 STAR: π , K S, K*, , ρ PHENIX: η, ω, J/  ccbar: star measured cross section as input; STAR Collaboration, PRL94(2005)062301 bbar/ccbar ratio from PYTHIA

14. ω invariant yields comparison in p+p collisions PHENIX results: arXiv:1005.3674 Z. Tang et al., PRC 79,051901(R)(2009) Tsallis fit is only the fit function of the previous published high pT ω yields reconstructed by hadron channel! 4/1/201014MRPC workshop at USTC, Lijuan Ruan (BNL)

15. 4/1/2010MRPC workshop at USTC, Lijuan Ruan (BNL)15 Di-lepton projection with y2010 Au+Au data STAR: low material budget, full Time of Flight coverage beam energies (Au+Au) minbias events (million) ω  ee projection (S/B, significance) φ  ee projection (S/B, significance) 200 GeV3551/180, 27 σ1/120, 17 σ 62 GeV14315 σ10 σ 39 GeV25022 σ14 σ What can STAR do in the near future with data on disk: the low mass continuum, 0.15-0.7 GeV vector mesons: ρ, ω,   ee virtual photons intermediate mass continuum (thermal radiation) ccbar modification J/   ee at low p T …

16. 4/1/2010MRPC workshop at USTC, Lijuan Ruan (BNL)16 Charm Contribution to Di-lepton Spectrum at RHIC PHENIX, PRC81,034911(2010) 1.Charm contribution to di-lepton spectra is significant at low mass at RHIC. 2.Charm contribution to di-lepton spectra might be dominant in the intermediate mass region. Its correlation makes a big difference to access the thermal radiation contribution from QGP.

17. 4/1/2010MRPC workshop at USTC, Lijuan Ruan (BNL)17 Distinguish heavy flavor and initial lepton pair production: e-muon correlation e  correlation simulation with Muon Telescope Detector 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., Journal of Physics G: Nucl. Part. Phys. 36 (2009) 095001 NA60, PRL100,022302(2008) R. Rapp, hep-ph/0010101

18. 4/1/2010MRPC workshop at USTC, Lijuan Ruan (BNL)18 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 MeasurementsJ/psi  ee, Upsilon  ee J/psi R AA, v 2 vs. p T &N part, Upsilon R AA vs. N part di-electron continuum di-electron spectra vs. p T FY12: first e-µ measurement FY13: J/psi  µµ, Upsilon  µµ J/psi R AA,v 2, vs. p T & N part, first look at different Upsilon states di-muon continuum B  J/psiX  µµX Precise measurements on different Upsilon states R AA versus N part, J/psi  µµ R AA, v 2 versus p T & N part, di-muon continuum, e-µ, D v 2 and R AA PhysicsColor screening features, quarkonia production mechanisms, vector meson in-medium modifications, low mass enhancement, Intermediate mass Better understanding on 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

19. 4/1/2010MRPC workshop at USTC, Lijuan Ruan (BNL)19 Summary STAR has a bright future on the di-lepton program. Di-lepton continuum measured in 200 GeV p+p collisions, cocktail simulation is consistent with data. Near term in Au+Au with the TOF: low mass continuum vector meson in-medium modifications virtual photons intermediate mass thermal radiation ccbar modification quarkonia rare decay and exotics … Long term with the MTD: e-muon correlation to distinguish heavy flavor production from initial lepton pair production di-muon continuum …

20. Search for rare decay  e + e - : an example  e+e- PDG BR <2.7x10 -5 SM: ~10 -9 New physics: ~10 -8, 10 -7 RHIC: ideal for such search; >10 11  annual yield at |y|<1; Clean low-pt electron identification ~10 7  STAR Simulation 4/1/201020MRPC workshop at USTC, Lijuan Ruan (BNL) CLEO Collaboration, Phys.Rev. D 56, 5359 (1997); CELSIUS/WASA Collaboration, arXiv:0711.3531, Phys.Rev. D 77, 032004 (2008); A.E. Dorokhov, arXiv:0905.4577; J.D. Bjorken et al., Phys.Rev. D 80, 075018 (2009); M.J. Savage, M.E. Luke and M.B. Wise, Phys.Lett. B 291, 481 (1992); L. Bergstrom, Z. Phys. C 14, 129 (1982).