The STAR Experiment Texas A&M University A. M. Hamed for the STAR collaboration 1 Quark Matter 2009 Knoxville, TN

Outline  Motivation  New Results  d+Au  enhanced kinematic reach  Status QM08 and HP08  summary 2  Analysis Technique

O(  α s ) 3 Energy loss via  -jet coincidence Ahmed M. Hamed QM2009 Knoxville, Tennessee Higher order processes: kT Photon Bremsstrahlung O(  α s 2 ) Example of Bremsstrahlung diagrams LO processes o E  = E parton, modulo correction from initial-state radiation. Motivation: o Accurate measurements of energy dependence of energy loss (  -jet in AuAu and pp) o Possible measurements of path length dependence of energy loss (  -jet vs  0 -jet)  hh hh 00 P (  E)  f (E, L, C R,  s ) Energy loss functional form

4 Away side yield per direct  vs. per  0 Away side of direct  trigger in the dominant channel is always quark vs. quark/gluon mix in  0 trigger 2. Away side parton type a. Gluon jet fragment softer than quark jet at the same energy b. Gluon is bi-colored (C A =3, C F =4/3) “color factor” Ahmed M. Hamed QM2009 Knoxville, Tennessee 3. Initial parton energy  0 -triggers result from higher-energy parton than  -triggers with same measured energy. Associated away-side yield per  0 trigger is higher than that of  trigger at the same energy 1. Geometrical bias  is electromagnetically interacting particle, then  0 -triggers are surface biased compared to direct . On average the away side of  0 trigger suffers from longer path length. Medium vacuum

 -  5 Build correlation function for neutral “triggers” with “associated” charged particles Use transverse shower profile to distinguish 2-photon “  0 ” from single-photon showers “  rich ” 00  rich Analysis technique Ahmed M. Hamed QM2009 Knoxville, Tennessee -  extraction Y  +h  The fact that direct photon has zero near side yield assuming all sources of background have similar correlations to that of symmetric decay  0.

6  hh What we measure? Y  +h  D=Y  +h  / N  ~37 0 Conditional yield per trigger p t / p t hh  FF Away-side 00 hh Y  0 +h  D=Y  0+h  / N  0 p t / p t hh  D vacuum D medium I AA 1 I AA >1  I AA =1  I AA <1  Away-side For direct  : Energy dependence of energy loss How about path length dependence? I AA  00 ? (zT)(zT) (zT)(zT) Same path length “tangential emission” Punch through Ahmed M. Hamed QM2009 Knoxville, Tennessee To reduce the punch through: low z T I AA  00 = (zT)(zT) (zT)(zT) Zhang et al. arXiv:

7 Status QM’08 and HP’08 The associated yield with direct  trigger agrees with theoretical model predictions. I AA, I AA, I AA, R AA and R AA 0h0h h  hh  h h h   h h 00 Similar suppression in Ahmed M. Hamed QM2009 Knoxville, Tennessee Direct  – charged hadron coincidence for p T, assoc > 3 GeV/c 00 Direct 

8 New Results :  0 -Triggered yields Good agreement between data from different runs and different systems at STAR detector The first measurements at RHIC energy for FF cover this wide range of kinematics in pp and dAu of  0 -hadron coincidence Ahmed M. Hamed QM2009 Knoxville, Tennessee Yields show significant medium effect D(z T ) = Y  0 +h  N0N0 pp, run’06 AuAu, run’07 dAu, run’ ub pb -1 Data: 34 nb -1

9 New Results :  0 -Triggered I AA Ahmed M. Hamed QM2009 Knoxville, Tennessee The per trigger yield suppression is final state effect. Quantifying the suppression  -jet coincidence measurements

10 New Results : Direct  -Triggered yields Ahmed M. Hamed QM2009 Knoxville, Tennessee The first measurements at RHIC energy for FF cover this wide range of kinematics in pp and dAu of  -hadron coincidence within this level of precision Yields show significant medium effect D(z T ) = Y  +h  NN FF from  -jet coincidence can be described by theory within current uncertainties (Theory-Data)/Theory

11 New Results : Direct  -Triggered yields in dAu Ahmed M. Hamed QM2009 Knoxville, Tennessee The modification in the FF of  -jet coincidence measurements is final state effect Associated yield for direct photons is significantly suppressed compared to that of  0 due to the difference in the parton initial energy. Good agreement between pp and dAu data for  and  0 results

12 New Results : I AA for  and  0 theory vs. data Ahmed M. Hamed QM2009 Knoxville, Tennessee Different trends in various theoretical calculations, and no strong rise on I AA (data)  at Z T < 0.6 as expected by one calculation. No difference between I AA  and  0 results within measured uncertainties. Path length energy loss dependence can also be explored by  -jet in- and out- of the reaction zone.

13 New Results : Direct  -Triggered I AA vs. centrality Ahmed M. Hamed QM2009 Knoxville, Tennessee Theory describes the centrality dependence of the direct photon-triggered I AA within the current uncertainties. The p T dependence (also in other centralities) does not support the volume vs. surface emission picture within the measured range of p T associated.

14 Summary An accurate measurement for the energy dependence of energy loss is reported via  -jet coincidence measurements. Continuous effort is currently being undertaken to improve the uncertainty levels and background contributions.  -jet coincidence measurements with respect to the reaction plane are needed to explore the path length dependence of the energy loss. The similarity between  -jet coincidence and  0 -jet coincidence measurements at intermediate z T indicates the need to push to lowest z T region. Ahmed M. Hamed QM2009 Knoxville, Tennessee

15 Thank you for your attention. You feedback is highly appreciated Thank you Ahmed M. Hamed QM2009 Knoxville, Tennessee