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J/ ψ measurements in Au+Au and p+p Collisions at 200 GeV

J/ ψ measurements in Au+Au and p+p Collisions at 200 GeV. Johan Gonzalez for the STAR Collaboration. Motivation J/ ψ in Au+Au J/ ψ in p+p Conclusions. Outline. Motivation. J/ Ψ.

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J/ ψ measurements in Au+Au and p+p Collisions at 200 GeV

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  1. J/ψ measurements in Au+Au and p+p Collisions at 200 GeV Johan Gonzalez for the STAR Collaboration • Motivation • J/ψ in Au+Au • J/ψ in p+p • Conclusions Outline Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  2. Motivation J/Ψ • Charm quarks are primarily produced in the initial stages of the collision via hard scatterings  Can be used to probe the initial conditions of the hot, dense matter produced • Screening and scattering mechanisms predicted[1] to destroy the J/ψ in the Quark-Gluon Plasma • This leads to a suppression of J/ψ production, which would imply deconfinement • On the other hand, regeneration models[2,3,4] predict an enhanced production of hidden charm states for sufficiently high charm densities • This would imply thermalization of charm quarks, and by extension, the light quarks that comprise the QGP • Scrutiny of the centrality dependence and transverse momentum systematics of charmonium production can help distinguish between the two scenarios regeneration [3] Thews, Schroedter, Rafelski, Phys. Rev. C 63, 054905 [4] P.B.M., Andronic, hep-ph/0402291 [1] T. Matsui and H. Satz, Phys. Lett. B 178 (1986) 416. [2] Grandchamp, Rapp, Brown hep-ph/0403204 Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  3. Motivation An illustration of the competition of regeneration and suppression mechanisms • Suppression mechanisms§, such as Debye screening or gluon scatterings, destroy the J/ψ. Magnitude of suppression depends on collision energy and centrality • Regeneration mechanisms, such as Statistical Hadronization, are expected to contribute in conditions of high parton densities • The two scenarios are not mutually exclusive – regeneration mechanisms may be at play even in the case where only a suppression is observed L. Grandchamp and R. Rapp, Phys. Lett. B523 60 (2001) regeneration suppression § These exclude “normal” suppression mechanisms, such as collisions with co-movers Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  4. The STAR Detector Schematic side view of the STAR detector • For the Au+Au analysis in run 4, we only used the TPC • For the p+p analysis, we used the TPC in conjunction with the BEMC. Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  5. J/ψe+e- in Au+Au @ 200GeV Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  6. Data & Analysis Cuts • Dataset • Run IV Au+Au collisions • Trigger selection: Minbias • Analysis Cuts • |vertexZ|<30cm • |η| < 1.0 • p > 500 MeV/c • Selected electron band, skipped hadron bands • Scanned 20M minbias events12M 0-80% events after cuts selected tracks dE/dx p (GeV/c) Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  7. Dielectron Invariant Mass Spectra J/ψe+e- (BR = 5.93%) STAR Preliminary Zoom Same event Mixed event • The dielectron invariant mass spectra are generated using the event mixing technique • Subtracted spectrum shows a peak of 3.5 sigma significance in the vicinity of the J/ψ mass 0-80% 12M events STAR Preliminary Subtracted spectrum Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  8. Dielectron Invariant Mass Spectra STAR Preliminary • The invariant mass spectra in the 0-20, 20-40, 40-80, and 0-80% centrality classes are shown • Number of J/ψ in each centrality class is determined by bin counting • Gaussian fits (widths are held fixed to the value seen in minbias events) are used to estimate systematics • Signal in the 0-20% bin is rather weak, so only an upper limit is quoted Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  9. Centrality Dependence of Scaled J/ψ yields • The Nbin-scaled yields are plotted vs number of participants • Bars indicate statistical uncertainties and the bands indicate the systematic uncertainties • An upper limit is quoted for the most central bin STAR Preliminary 0-80% Binary Scaling (black line and grey band) Determined from PHENIX data (nucl-ex/0507032) Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  10. Centrality Dependence of Scaled J/ψ yields STAR Preliminary • Statistical Hadronization[1] calculations are shown for various values of the differential c-cbar cross section • The model appears to overpredict the scaled J/ψ yields for most values of the c-cbar cross section • However, it should be noted that the uncertainties in the measured[2,3] c-cbar cross sections are rather large at this time. • Statistical Hadronization Synopsis: • Complete screening of primordial J/ψ’s • J/ψ’s regenerated at chemical freezout from thermalized c-cbars [1] A. Andronic et al., Phys.Lett. B571 (2003) 36-44 [2] PHENIX, Phys.Rev.Lett. 96 (2006) 032001 [3] STAR, Phys. Rev. Lett. 94 (2005) 062301 pQCD x-section from:Cacciari, Nason, Vogt, PRL 95 (2005) 122001 Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  11. J/ψe+e- in p+p @ 200GeV Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  12. Data & Analysis Cuts A typical triggered event • Dataset • Run V p+p collisions • Trigger selection: J/ψ • Level-0 portion require 2 BEMC hits above 1.2 GeV with Δφ> 60o • Level-2 portion acts as a photon veto and also makes an online invariant mass cut of 2<m<15 GeV (m2~2*E1*E2*(1-cos(θ))) • Scanned 1.6M events • Analysis Cuts • |vertexZ|<100cm • TPC tracks were required to match with BEMC towers • 0.15 < p/Etower < 2.0 • 3.2 < dE/dx < 5.0 keV/cm Track selection Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  13. Dielectron Invariant Mass Spectrum • The (raw) background subtracted dielectron invariant mass spectrum is shown • A signal of 5 sigma significance is clearly visible • The trigger has been successfully commissioned and is currently running as we speak STAR Preliminary J/ψe+e- in p+p @ 200 GeV More statistics are expected from Run 6 Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  14. Summary & Conclusions • We have investigated the production of J/ψe+e- in Au+Au and p+p collisions at 200 GeV • Taking the STAR measured c-cbar cross section, our Au+Au results rule out a model of J/ψ production via Statistical Hadronization • A J/ψ trigger in p+p collisions was successfully tested and is now being implemented in this year’s run • With the future inclusion of the Barrel Time of Flight detector, electron identification—and by extension J/ψ reconstruction—will be substantially augmented in STAR • In future endeavors, we will carry out a detailed analysis of the centrality and transverse momentum dependence of J/ψ in order to fully disentangle regeneration and suppression scenarios Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  15. Extra slides Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  16. Efficiency Corrections (1/3) TPC+TOF • Embedded simulated J/ψ’s into “Run 4-like” events: • Full field • |vertexZ|<30cm • J/ψe+e- only • Flat in pT and rapidity • Quantities having to do with dE/dx are not well simulated  must use data itself to correct spectra: • Isolated electron band using TOF data in order to determine number of pairs lost due to nSigma cuts • Used the correlation* between nHits and nHitsDedx to assess number of pairs lost due to nHitsDedx cuts • Folded data-determined corrections into embedding efficiency analysis Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  17. Efficiency Corrections (2/3) 20-40% 0-20% 40-80% 0-80% Centrality dependence of efficiency corrections • Efficiency corrections are shown as a function of transverse momentum • Strong centrality dependence is observed • Sag in the vicinity of 1.5 GeV/c induced by nSigma cuts Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  18. Efficiency Corrections (3/3) • Correction factors were obtained by embedding J/ψ’s into real data • Due to lack of statistics, we had to resort to numerical integration • Randomly sampled J/ψ candidates and carried out correction: N dN dy Σ ( ) 1 ε(pT) = Where N is the number of candidates in a given centrality bin and ε is the efficiency Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  19. Systematic Uncertainties Error Budget • Varied kinematic and track quality cuts in embedding. Variation of nHitsDedx cuts done in parallel with nHits variation • Assessed uncertainty in correction the correction technique • Uncertainty in background subtraction determined to contribute about 10% to overall errors, assumed equal across all centralities • The uncertainty in the yield due to nSigma cuts is the dominant contributor to systematic errors, accuracy limited by TOF statistics Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

  20. Comparison with PHENIX Results Comparison with PHENIX Run 2 Comparison with PHENIX Run 4 • Our data is similar to PHENIX Run 2 data • Our results are higher than PHENIX Run 4 data, but error bars should be taken seriously! Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006

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