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The di-lepton physics program at STAR

The di-lepton physics program at STAR. L ijuan Ruan for the STAR Collaboration ( Brookhaven National Laboratory ). Outline: Motivation and Introduction Recent results from STAR Future perspectives from STAR Conclusions and outlook. S. Bass. Probe the medium properties.

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The di-lepton physics program at STAR

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  1. The di-lepton physics program at STAR LijuanRuan for the STAR Collaboration (Brookhaven National Laboratory) Outline: • Motivation and Introduction • Recent results from STAR • Future perspectives from STAR • Conclusions and outlook Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  2. 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(2008)022302 NA60, Eur.Phys.J.C59(2009)607 Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  3. STAR detector: TPC & MRPC-TOF • A recent technology (TOF) ---- • Multi-gap Resistive Plate Chamber • Good timing resolution, • (, K) < 1.6 GeV/c, proton < 3 GeV/c • Coverage: -0.9<<0.9 • Coverage • (72% y2009, 100% y2010) Time Projection Chamber • Tracking • Ionization energy loss (dE/dx): • (, K) < 0.7 or > 3 GeV/c, • proton < 1 or > 3 GeV/c • Coverage -1<<1 Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  4. STAR Time of Flight detector performance |1/-1|<0.03 STAR, PLB616(2005)8 STAR, PRL94(2005)062301 200 GeV d+Au collisions 200 GeV d+Au collisions TOF PID: (, K) ~ 1.6, proton ~ 3 GeV/c STAR Collaboration, PLB616(2005)8 TOF enables clean electron PID up to PT < 3 GeV/c. STAR Collaboration, PRL94(2005)062301 M. Shao et al., NIMA 558(2006)419 Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  5. Di-electron signal 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- X Drell-Yan contribution In Au+Au collisions, we search for QGP thermal radiation at 1.1<Mee<3.0 GeV/c2(intermediate mass range) Vector meson in-medium modificationsat Mee<1.1 GeV/c2(low mass range) Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  6. Di-electron production in 200 GeV p+p 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/c2) Simulation: charm correlation contribution is from PYTHIA .   J/ STAR acceptance: |yee|<1, |ηe|<1, pT>0.2 GeV/c STAR Collaboration, arXiv:1204.1890. Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  7. Di-electron angular correlation in 200 GeV p+p STAR Collaboration, arXiv:1204.1890. Taking advantage of 2 azimuthal coverage, with better statistics in future, we will do the angular correlations of di-leptons from ccbar contribution. Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  8. Di-electron spectra in 200 GeV Au+Au 0-80%Au+Au 0-10%Au+Au     J/ J/ possible enhancement factor in 0.15<Mee<0.75 Gev/c2 See J. Zhao’s talk for detail PHENIX results, see A. Drees’ talk. Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  9. 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: <β>=0 in p+p, <β>=0.47 in 0-80% AuAu. Z.Tang et al., PRC79(2009)051901 STAR, QM2011, SQM2011 Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  10. 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. STAR Preliminary STAR, SQM2011 Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  11. Di-lepton elliptic flow • Low pT direct photon elliptic flow measurement could provide direct constraints on QGP dynamics (η/s, T, t0…). • Excess of direct photon yield over p+p and substantial positive v2 observed at pT<4 GeV/c in central Au+Au, suggestive of significant contribution from the hadronic sources at later stage. (H. v. Hees, C. Gale, R. Rapp, PRC84(2011)054906) • Di-lepton v2 versus pT & Mll: probe the properties of the medium from hadron-gas dominated to QGP dominated. (R. Chatterjee, D. K. Srivastava, U. Heinz, C. Gale, PRC75(2007)054909) PHENIX, arXiv: 1105.4126 PHENIX: PRL104 (2010)132301 K. Dusling NPA839(2010)70 pT (GeV/c) Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  12. v2 versus Mee at 200 GeV Au+Au STAR Preliminary A factor of 2 more minbias data from run11 are on disk. 0-80% Au+Au at 200 GeV STAR, SQM2011, DNP2011 Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  13. v2 of di-electrons from π0 and η Dalitz decay 0-80% Au+Au at 200 GeV STAR Preliminary 0-80% Au+Au at 200 GeV STAR Preliminary 0: PHENIX, PRC 80(2009)054907 To provide a comparison (blue) curve from simulation: 1. Parameterize  meson v2 results 2. Do the Dalitz decay simulation and obtain expected v2 of di-electrons pairs from 0 Dalitz decay. 3. This is consistent with our di-electrons v2 results. • To provide a comparison (blue) curve from simulation: • Assume η v2 same as KS • do the same Dalitz decay • procedure STAR, SQM2011, DNP2011 Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  14. v2 of di-electrons at 0.5<Mee<1.1 GeV/c2 0-80% Au+Au at 200 GeV 0-80% Au+Au at 200 GeV STAR preliminary STAR preliminary The elliptic flow of ee pairs in the  mass region is consistent with the result of KK within large errors KK: STAR, PRL99(2007)112301 STAR, DNP2011 Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  15. Di-electron results in 39 GeV Au+Au Raw, efficiency correction to be done. STAR, CPOD2011 Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  16. To the future STAR has started the di-lepton program with a bright future enabled by the TOF upgrade : • Di-electron spectrum in 200 GeV p+p collisions: Cocktail simulation consistent with data Charm correlation contribution dominates at intermediate mass • Di-electron in 200 GeV Au+Au collisions: A possible low mass enhancement with respect to cocktail expectation at Mee 0.15-0.75 GeV/c2 ee shows a similar flow velocity at freeze-out as light hadrons  yields are similar between di-leptonic decay and hadronic decay channel Di-electron v2 from low to high mass measured Towards the future: Differential measurements (Mee, pT, v2) are on-going Energy dependence (19-200 GeV) can be systematically studied at STAR Precise open charm measurements help study heavy flavor dynamics with HFT e-µ correlation (spectrum and v2) to distinguish heavy flavor production from initial lepton pair production with MTD upgrade Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  17. The Heavy Flavor Tracker (HFT) SSD • The HFT puts 4 layers of Silicon around the vertex • Provides ~20 m space point resolution on tracks • Works at high rate (~ 800 Hz – 1K) • Does topological reconstruction of open charm • Will be ready for the 2014 run IST 30 Beampipe Pixel Detector 20 10 -10 -20 -30 Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  18. The HFT – study heavy flavor dynamics HFT CDR RCP, v2, baryon/meson ratios: heavy quark transport co-efficients, energy loss mechanism, thermalization with medium. To understand heavy quark dynamics in the medium, constrain model inputs to calculate di-leptons from heavy flavor correlation. Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  19. Concept of design of the STAR-MTD Multi-gap Resistive Plate Chamber (MRPC): gas detector, avalanche mode A detectorwith long-MRPCs covers the whole iron bars and leave the gaps in- between uncovered. Acceptance: 45% at ||<0.5 118 modules, 1416 readout strips, 2832 readout channels Long-MRPC detector technology, electronics same as used in STAR-TOF MTD Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  20. Measure charm correlation with MTD upgrade: ccbare+m Z. Xu, BNL LDRD 07-007; L. Ruan et al., JPG36 (2009) 095001 R. Rapp, hep-ph/0010101 e correlation simulation with Muon Telescope Detector (MTD) at STAR from ccbar:S/B=2 (Meu>3 GeV/c2 and pT(e)<2 GeV/c) S/B=8 with electron pairing and tof association MTD: construction has started in FY2011; project completion in FY2014 Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  21. Simulation precision projection: ccbare+m Red: PYTHIA ccbareµ Black: de-correlated + energy loss PYTHIA ccbareµ with electron and muon coincidence trigger projection done with 43% MTD in place for 2013 For FY2013, request three weeks Au+Au run at 200 GeV: e-muon correlation  measure charm correlation modification Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  22. Di-lepton measurements at STAR Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  23. Summary • STAR has started the di-lepton program with a bright future enabled by the TOF upgrade : Di-electron spectrum from low to high mass measured in 200 GeV p+p and Au+Au collisions Di-electron v2 from low to high mass measured in 200 GeV Au+Au collisions Energy dependence (19-200 GeV) can be nicely and systematically studied at STAR Differential measurements (Mee, pT, v2) are on-going • Future with the HFT and MTD: Precise measurements on charmed hadrons help understand charm quark dynamics in the medium. e-µ correlation (spectrum and v2) to directly measure heavy flavor correlation contribution to di-lepton continuum  to access the QGP thermal radiation. Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  24. Backup Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  25. Compared to theoretical calculations within STAR acceptance HG_med, QGP from R. Rapp (private communication) R. Rapp and J.Wambach, Adv. Nucl. Phys. 25, 1 (2000). Blue dotted: HG_medium Pink dotted: QGP Solid lines: upper: cocktail + HG+QGP lower: cocktail Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  26. Data sets for di-electron analyses 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%) Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  27. Electron identification capability 200 GeV p+p collisions |1/β-1|<0.03 |1/β-1/βexpected|<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. Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  28. Di-electron signals in p+p and Au+Au STAR preliminary p+p @ 200 GeV Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  29. Di-lepton background Mixed-event normalized to like-sign within 0.4 - 1.5 GeV/c2 Cross-pair bg. can only be described by like-sign method. For minimum bias events, we subtracted like-sign bg at Mee<0.4 GeV/c2, mixed-event bg subtraction was applied at Mee>0.4 GeV/c2. Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  30. Di-lepton azimuthal angle distribution STAR preliminary STAR preliminary Mixed-event normalized to like-sign within 0.4 - 1.5 GeV/c2 Cross-pair bg. can only be described by like-sign method. For minimum bias events, we subtracted like-sign bg at Mee<0.4 GeV/c2, mixed-event bg subtraction was applied at Mee>0.4 GeV/c2. Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  31. Di-lepton background Mixed-event normalized to like-sign within 1 - 3 GeV/c2 Cross-pair bg. can only be described by like-sign method. For minimum bias events, we subtracted like-sign bg at Mee<0.4 GeV/c2, mixed-event bg subtraction was applied at Mee>0.4 GeV/c2. Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  32. Low mass enhancement PHENIX PRC 81 (2010) 034911 Enhancement factor in 0.15<Mee<0.75 Gev/c2 Note: Acceptance difference etc. Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  33. Systematic uncertainties p+p: Au+Au: STAR preliminary Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  34. Reproduce PHENIX cocktail • Reproduce the cocktail within PHENIX acceptance by our method. • The momentum resolution are from STAR. STAR preliminary Scaled by all the yields from PHENIX paper[1], we can reproduce the PHENIX cocktail. [1]. Phys. Rev. C 81, 034911 (2010). Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  35. Check with acceptance difference STAR preliminary STAR preliminary Acceptance difference: Scaled by the acceptance difference Scaled by same meson and charm yields. Difference at low mass is not from the simulation but from the measurements. Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  36. STAR data with PHENIX  acceptance STAR data with PHENIX  acceptance cut Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  37. v2 standard event-plane method ν2Total is flow of unlike-sign pairs. v2B(M) is flow of background calculated using the like-sign or mixed events pairs. v2S is flow of signal. NS is the signal number, NB is the background (like-sign) number. N(S+B) is unlike-sign number. Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

  38. Unlike-sign and background v2 STAR preliminary STAR preliminary Signal flow Unlike-sign v2 Background v2 Signal/(Signal+Background) Hard Probes 2012, May 27-June 1, Lijuan Ruan (BNL)

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