1 Probing the medium with photons Outline: oMotivation oExperiment oResults oConclusion oIntroduction LBNL Saskia Mioduszewski Ahmed Hamed

2 Probing the medium High-p T Spectra I – Light quarks and gluons  The suppression of  0, s and , s is very similar. suppression occurs at the parton level.  The binary scaling of direct photons is strong evidence that suppression is not an initial state effect. Mid-rapidity Statistical Method LBNL   s C x qL ^ 2 “Static medium” Gluons dominance at mid-rapidity at RHIC energy. Photons PHENIX, QM05

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

4 LBNL Baryons and Mesons  Clear meson-baryon yield differences at intermediate p T. Probing the medium High-p T Spectra III– quarks and gluons  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. STAR QM05 and nucl-ex/

5 LBNL ?  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 Probing the medium Jet-like azimuthal correlations LBNL  Surface bias for the trigger particle.  Trigger particle with no surface bias is required for better quantitative measurements of the away-side modifications. Charged hadrons

6 Introduction Summary LBNL  Better understanding for the energy loss mechanism! Elliptic flow. oDirect Photons: Gamma-charged hadrons correlation. 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:  Challengeable measurements! Doesn’t couple to the medium. QGP thermal photons. Test for binary scaling for hard process.

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

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

9 Experiment STAR BEMC Cross-section in  LBNL o120 modules. o4800 channels oSMDs: channels oPreShower: 4800 channels oLead-scintillator detector. oSampling calorimeter. oProjective towers. Cross-section in   BEMC face is ~2.2m away from the point of interaction at  =0. West side 0<  <1

10 Experiment Electromagnetic Shower LBNL  -plane  -plane High energy core. Low energy halo.  Electromagnetic transverse shower characteristics

11 Inclusive  - jet in Au+Au at  s=200GeV Thomas Dietel Quark Matter 2005 Results QM 2005 LBNL SIMULATION (pp)‏  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. STAR Preliminary  Simulation shows no associated particles in  -charged correlation.

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

13 Results Transverse Shower Profile LBNL  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.  Could be used to distinguish  0 /   Clear sensitivity to the halo region.

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

15 Conclusion LBNL   -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.

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