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Probing the medium with photons

This research outlines the investigation of the medium using photons as a probe. It covers the suppression of π0 and η, the behavior of heavy quarks, baryons and mesons, and jet-like azimuthal correlations in different collision experiments. The motivation, experimental setup, and results are discussed, highlighting the potential for a calibrated probe of the quark-gluon plasma.

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Probing the medium with photons

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  1. Probing the medium with photons Outline: • Introduction • Motivation • Experiment • Results • Conclusion Saskia Mioduszewski Ahmed Hamed LBNL 21-05-07

  2. Probing the mediumHigh-pT Spectra I – Light quarks and gluons Mid-rapidity Statistical Method Photons • The suppression of 0,s and ,s is very similar. suppression occurs at the parton level. PHENIX, QM05 • The binary scaling of direct photons is strong evidence that suppression is not an initial state effect. Gluons dominance at mid-rapidity at RHIC energy. 2 ^ <E>  sCxqL “Static medium” LBNL 21-05-07

  3. Probing the mediumHigh-pT Spectra II– Heavy quarks and gluons Non photonic electrons-Charm and Beauty • QCD is flavor independent, but heavy quarks at same pT are moving much slower than light quarks. Expected “dead-cone” with no induced gluon radiation. nucl-ex/0607012 • single-particle suppression in AuAu is strong evidence for the hot and • dense medium formation. • single-particle suppression does not constrain the mechanism of energy loss. LBNL 21-05-07

  4. Probing the mediumHigh-pT Spectra III– quarks and gluons Baryons and Mesons STAR QM05 and nucl-ex/0601042 • Clear meson-baryon yield differences at intermediate pT . • No reduction is observed in the baryon/meson ratio as expected in the gluon dominance picture. • Calibrated probe of the QGP is needed for better understanding of energy loss. LBNL 21-05-07

  5. Probing the mediumJet-like azimuthal correlations ? Charged hadrons • Near-side: p+p, d+Au, Au+Au is similar. • Back-to-back: Au+Au strongly suppressed relative to p+p and d+Au. Suppression of the back-to-back correlation in central Au+Au is a final-state effect • Surface bias for the trigger particle. • Trigger particle with no surface bias is required for better quantitative measurements of the away-side modifications. LBNL 21-05-07 LBNL 21-05-07

  6. Introduction Summary Four multipurpose experiments (BRAHMS, PHENIX, PHOBOS, STAR) • Empirical lines of evidence: Energy density well beyond critical value. Large elliptic flow. Jet quenching. dAu control experiment. • Interpreted in terms of a strongly coupled QGP and a new QCD state (?) Color Glass Condensate Required: • Better understanding for the energy loss mechanism! • Direct Photons: Doesn’t couple to the medium. • QGP thermal photons. • Elliptic flow. • Test for binary scaling for hard process. • Gamma-charged hadrons correlation. • Challengeable measurements! LBNL 21-05-07

  7. Motivation LODirect photons schematic view thermal: Decay photons hard: • single-particle suppression does not effectively constrain detailed energy-loss pictures. direct component Bremsstrahlung fragmentation component • Gamma-charged hadrons correlation. • Calibrated probe of the QGP – at LO. • No Surface Bias • Hard process • Possible candidate for quark/gluon jet discrimination at LO. LBNL 21-05-07

  8. ExperimentSTAR Detector • Tracker detectors(slow), Trigger detectors(fast), and Calorimeters(fast). • Measurements of hadrons production over a large solid angle. • STAR BEMC can probe for further higher transverse energy. LBNL 21-05-07

  9. Experiment STAR BEMC • Sampling calorimeter. • Lead-scintillator detector. • 120 modules. • Projective towers. • 4800 channels • SMDs: 36000 channels • PreShower: 4800 channels West side 0<<1 Cross-section in  Cross-section in  • BEMC face is ~2.2m away from the point of interaction at =0. LBNL 21-05-07

  10. Experiment Electromagnetic Shower -plane -plane • Electromagnetic transverse shower characteristics • High energy core. • Low energy halo. LBNL 21-05-07

  11. Results QM 2005 Inclusive g-jet in Au+Au at s=200GeV SIMULATION (pp) STAR Preliminary Thomas Dietel Quark Matter 2005 • Simulation shows no associated particles in -charged correlation. • The background is higher for central events. • Away-side decreases with increasing centrality. • Decrease in near-side due to the increased fraction of prompt photons. • Need /0 discrimination. LBNL 21-05-07

  12. Results QM 2006 pp at s=200GeV STAR Preliminary S. Chattopadhyay Quark Matter 2006 • Reduction in near angle peak towards photon Bin. • Effect is more prominent for larger Ettrigger . • Away-side yield is reduced. LBNL 21-05-07

  13. Results Transverse Shower Profile • Clear structure for the two showers in  and 0 at moderate energy. • 0 shower at high energy is still wider than the single photon shower. • Clear sensitivity to the halo region. • Could be used to distinguish 0/ LBNL 21-05-07

  14. ResultsRaw correlation function 0 Et_trg>12GeV 1 /c  /c Ntrg (rad) (rad) cucu 0-10% Et_trg>6GeV /c /c (rad) (rad) dN Y-axis: d () • Similar Away-side for 0 and  Preliminary • Reduction in the near-side for  compared to 0. • Reduction is more noticeable at higher Et_trg and also at higher centrality bins. LBNL 21-05-07

  15. Conclusion • -charged hadrons correlation is very promising tool for better understanding of the medium. • Shower shape study is required for direct photons identification. • Promising study for transverse shower profile is undertaken. • Comparison with the previous study of transverse shower profile is necessary. LBNL 21-05-07

  16. Thanks to all STAR Collaborators • Thanks to Texas A&M nuclear physics group. • Thank you all

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