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Direct Soft Photon Production in Au+Au Collisions at √s NN = 200 GeV: First Measurements at STAR

Direct Soft Photon Production in Au+Au Collisions at √s NN = 200 GeV: First Measurements at STAR. `. Outline. Object of Study Analysis Physics Goals & Setup MC studies Raw data Corrections Current Results Outlook and Future Plans. Initial hard scattering and pre-equilibrium photons.

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Direct Soft Photon Production in Au+Au Collisions at √s NN = 200 GeV: First Measurements at STAR

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  1. Direct Soft Photon Production in Au+Au Collisions at √sNN = 200 GeV: First Measurements at STAR `

  2. Outline • Object of Study • Analysis • Physics Goals & Setup • MC studies • Raw data • Corrections • Current Results • Outlook and Future Plans

  3. Initial hard scattering and pre-equilibrium photons Object of Study: What is meant by “direct photon”? Photons are produced in all stages of relativistic heavy-ion collision: from initial hard scattering to final hadronic state Thermal Radiation from QGP and Hadron Gas Hadron Decays Direct For simplicity we shall distinguish two sources only: (1) an early one in which photons are produced in the initial parton cascade leading to thermalization of a quark-gluon plasma and (2) a later one where photons are radiated from thermalized matter the quark-gluon plasma phase (QGP) and the hadronic phase (HG). Direct photons: sum of the prompt +pre-equilibrium and thermal (QGP+HRG) components.

  4. RHIC Object of Study: What is meant by “soft”? Direct photons in ultra-relativistic collisions of heavy nuclei were observed by WA98 experiment at CERN SPS and PHENIX experiment at RHIC. There is no experiment covering the soft photon region! No experiment covers PT 40 ... 1000 MeV ! WA 98 :Yield of Direct Photons Nuclear Theory, nucl-th/0503054

  5. Physics goals and setup Primary physics goals: • Direct photon cross-section • Inclusive photon cross-section : AuAu, dAu • Direct photon ratio (to better distingwish direct photon signal excess) : AuAu/dAu Current experimental setup: • pT range : 40 - 160 MeV/c photons, || < 1.0 • Collision type : AuAu 200, dAu 200 • All measurements on the same detector setup, systematics effect is minimized

  6. MC studies : low pT primary photons Inclusive Photons = Direct + Decayed background signal Background needs to be evaluated using Simulation Models: Photon's Yield in Simulated Collisions • Au+Au 200GeV Hijing 1.382 • d+Au 200GeV Hijing 1.382 • p+p 200GeV Pythia 6.203 • Decay photons from primary vertex (collision) • pT < 0.5 GeV • || < 1.0

  7. MC studies: Au+Au, d+Au, p+p collisions • AuAu200, dAu200 : simulated with Hijing 1.382 + GSTAR (propagation throughout the STAR detector) • pp200 : simulated with Pythia 6.203 + GSTAR • Decay photons from primary vertex • pt < 0.5 GeV • || < 0.5 • M = 0.5 T Photons resulting from 0,  and other hadronic decays (, , etc) are considered pt< 0.2 GeV : most of background comes from 0 ~ 90% --> 0 less 10% -->  less 2% --> cocktail

  8. MC studies (continuation): background photon yield at various centralities (Au+Au 200 GeV) Nuclear Geometry and Centrality Selection Combined bins often used: Centrality Nch_range 0-10%520 and up 20-60% 57...319 0-80% (MinBias) 14 and up Centrality 20-60 % (Nch)

  9. MC studies: d+Au, p+p background photon yield d+Au p+p

  10. Au+Au / d+Au background photon yield ratio AuAu 20-60% central AuAu 0-10% central

  11. STAR Experiment: Photon Reconstruction Setup Method: photon conversion on TPC gas Cuts used in analysis: • dAu z-vertex < 50.0 cm • AuAu z-vertex < 30.0 cm • |Nsigma (electron)| < 3.0 • DCA between tracks < 1.5 cm • Opening angle < 0.2 radian • Fit points > 10 Photons are reconstructed via e+e- conversion in STAR Time Projection Chamber (TPC). Photon Momentum reconstruction was done by the measured momenta of e+e- pairs produced from conversions. Excellent TPC PID capabilities : 99.99% pure electron sample up to 0.08 GeV using dE/dx over momentum cuts.

  12. STAR Experiment: Can we see soft photons? Extensive QA done: d+Au, Au+Au nice opening angle, invariant mass, DCA distributions showing great photon reconstruction! AuAu statistics: • ~5M AuAu 200 used • 0.5M events passed through cuts (z-vertex, MinBias trigger) • ~1K photons extracted dAu statistics: • ~25M dAu 200 used • 10.5M events passed through cuts (z-vertex, MinBias trigger) • ~1K photons extracted Corrected for pT-dependent reco efficiency using AuAu and dAu embedding data. Parameters: • Pseudorapidity : [-1, 1] • Electron pT : [0.02MeV, 0.08Mev] We see enough already, 50% of AuAu dataset is still to be used in analysis!

  13. Experiment: difficulties encountered TPC conversion radius: • MC is different from real data Conversion efficiency: • Embedding shows higher conversion rate than real data Real photon (reconstructed e+e- pair) conversion radius MC photon (reconstructed e+e- pair) conversion radius

  14. Experiment:photon reconstruction efficiency evaluation using embedding An embedded MC photon is considered found, if it is converted into e+e- pair (at a particular distance from the collision center limited within the TPC volume) and both e+ and e- tracks were reconstructed. • eff_pT = [ (Reco number of MC photons) / (Total number of MC photons) ]*100% at particular ptbineff_ = [(Reco number of MC photons) / (Total number of MC photons) ]*100% at particulat  bin • The momentum resolution reflects the quality of the track reconstruction. The resolution was also determined by embedding MC simulated particle (photon) into real events. It shows momentum divergence between MC track and associated reconstructed track. A MC photon reconstruction assumption is the same as for the efficiency. Eta efficiency AuAu200 Pt Efficiency dAu200 dAu200

  15. Experiment: momentum resolution

  16. While work is still in progress,... STAR preliminary AuAu200 inclusive photons yield over dAu200 inclusive photons yield ratio

  17. Raw inclusive dN/dP yields: d+Au, Au+Au Only the basic corrections applied!

  18. Outlook • While work is still in progress, first results are quite promising! • AuAu/dAu ratio is ready, more work on simulation needs to be done to understand the phenomena: • Hijing tuning • Other models needs to be considered • High hopes on the rest of dataset. • STAR upgrades may provide new ways to explore low-pT photons.

  19. Future: STAR upgrades Crystal Detector R&D Proposal for Soft Photon Measurement in STAR The proposed soft photon program covers photon transverse momentum range of 20 MeV to 1000 MeV at mid-rapidity. Physics program will focus on three aspects: 1) thermal photon radiation from the Quark Gluon Plasma (QGP) phase in nucleus-nucleus collisions at RHIC; 2) measurement of radiative decays of particles such as 0 and c states and explore spectroscopy of particles with soft photon and 0 in the final state; 3) soft photon radiations from bulk quark matter through processes such as jets traversing dense partonic matter, radiation from collective high angular momentum state and possible synchrotron radiation from high longitudinal E/B field from non-equilibrium stage during the transition from the Color Glass Condensate to Quark-Gluon Plasma.

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