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Hard Probes 2008, June 8-14th, Galicia, Spain

The STAR Experiment. Direct  -charged hadron azimuthal correlation measurements. Hard Probes 2008, June 8-14th, Galicia, Spain. A. Hamed. Texas A&M University. Hard Probes 2008. Outline. Discussion on energy loss observables. Analysis technique outline. Direct  results.

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Hard Probes 2008, June 8-14th, Galicia, Spain

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  1. The STAR Experiment Direct -charged hadron azimuthal correlation measurements Hard Probes 2008, June 8-14th, Galicia, Spain A. Hamed Texas A&M University Hard Probes 2008

  2. Outline • Discussion on energy loss observables • Analysis technique outline • Direct  results • Luminosity projection • Summary and outlook

  3. d2NAA/dydpT TAA . d2/dydpT There is no single commonly accepted calculation of the underlying physics to describe in-medium energy loss for different quark generations as well as for the gluon. But nature cannot realize contradictions. When our physical theories lead to paradox we must find a way out. Paradoxes focus our attention, and we think harder. F. Wilczek “Nobel Lecture 2004” = RAA of light quarks, heavy quarks and gluons! PRL. 96, 202301 (2006) PRL 98 (2007)192301 STAR QM08 • RAA of light quarks is pt independent as expected by the radiative energy loss. Paradoxes • Unexpected level of suppression for the heavy quarks. • No sign for the color factor effect on energy loss. A. Hamed STAR Experiment HP2008 Spain 8-14th June

  4. Access to parton initial energy  Direct photon + jet provides probe to measure the fragmentation function IAA from di-hadron “charged” azimuthal correlations PRL 98 (2007) 212301 PRL 97 (2006) 212301 , Direct comparison to the fragmentation functions in vacuum and in medium Model dependent calculations show that IAA is more sensitive than RAA but both have diminished sensitivity at high gluon density. The ratio DAuAu/DdAu is independent of zT for zT > 0.4 and has similar magnitude to RAA. A. Hamed STAR Experiment HP2008 Spain 8-14th June

  5. Extract the yields associated with direct photon triggers Analysis technique • Build correlation function for neutral “triggers” with “associated” charged particles • Use transverse shower profile to distinguish 2-photon from single-photon showers • Comparison of 0 – triggered yields with previously measured charged-hadrons- triggered yields. A. Hamed STAR Experiment HP2008 Spain 8-14th June

  6. Analysis technique BEMC TPC Eγ= Eparton  2 Beam axis 0 180° Eπ‹ Eparton 0 Associated charged particles No track with p > 3 GeV/c points to the trigger tower Analysis technique • Correlate photon candidate “triggers” with associated tracks STAR Detector Use  triggers to explore fragmentation functions in p+p and Au+Au A. Hamed STAR Experiment HP2008 Spain 8-14th June A. Hamed STAR Experiment HP2008 Spain 8-14th June

  7. Use Transverse Shower Profile to distinguish between /0. STAR BEMC and BSMD Cross section in  The two photons originated from 0 hit the same tower at pT>8GeV/c 0.1 A. Hamed STAR Experiment HP2008 Spain 8-14th June

  8. On the Transverse Shower Profile Cut ∑i i ri1.5 Etotal Shower shape cut for 0 Shower shape cut for  Very pure sample of 0  selection is not tight  rich sample • Two photons (0) produce a more diffuse shower than single photons () • Wider shower has small value of such quantity  7 RM Two dimensional shower shape 0 i : strip energy ri : distance relative to energy maxima A. Hamed STAR Experiment HP2008 Spain 8-14th june

  9. Trigger photons-charged particles azimuthal correlations STAR Preliminary • Near side is suppressed with centrality which might due to the increase of /0 ratio . A. Hamed STAR Experiment HP2008 Spain 8-14th june

  10. Effect of Shower Shape Cut • The -rich sample has lower near-side yield than 0but not zero. Applying Shower Shape Cut A. Hamed STAR Experiment HP2008 Spain 8-14th june

  11. Limitations of the shower shape cut Shower Shape Cuts: Reject most of the 0’s. But do not reject photons from: highly asymmetric 0 decay. 10% of all 0 with pT > 8 GeV/c ’s - similar level of background as asymmetric p0 fragmentation photons 10% of inclusive  at intermediate pT in p+p ~30-40% of direct  at PT > 8 GeV/c. A. Hamed STAR Experiment HP2008 Spain 8-14th June

  12. Comparison of 0-triggered yields to charged-hadron triggered yields • The zT dependence of per trigger yields in near- and away-side correlation peaks of 0-charged and charged-charged correlations are very similar. PRL 97 162301 (2006). A. Hamed STAR Experiment HP2008 Spain 8-14th June

  13. Centrality dependence of IAA of 0 triggers • IAA of the away-side in 0-trigger shows similar suppression for all pt associated bins (3-8GeV/c). A. Hamed STAR Experiment HP2008 Spain 8-14th June

  14. away away Y+h = (Y-rich+h - RY0+h )/1-R near near R=Y-rich+h/Y0+h Extract direct  associated yield Extraction of direct away-side yields • Assume no near-side yield for direct then the away-side yields per trigger obey This procedure removes correlations due to contamination (asymmetric decay photons+ fragmentation photons) with assumption that correlation is similar to 0 – triggered correlation. A. Hamed STAR Experiment HP2008 Spain 8-14th June

  15. Centrality dependence IAA of direct  triggers. "T. Renk and K. Eskola, Phys.Rev. C75:054910,2007" "X-N Wang & H. Zhang et al (to be published)" • Within the current uncertainty in the scaling due to p+p measurements, the IAA of direct  and 0 are similar. A. Hamed STAR Experiment HP2008 Spain 8-14th June

  16. Centrality dependence of ICP of direct  triggers • Icp of -jet exhibits similar suppression of the away-side yield per trigger for associated particles of 3-8GeV/c. A. Hamed STAR Experiment HP2008 Spain 8-14th June

  17. D(ZT) of direct  triggers vs. 0 triggers • The away-side yield per trigger of direct  triggers shows smaller value compared to 0 triggers. A. Hamed STAR Experiment HP2008 Spain 8-14th June

  18. D(ZT) of direct  triggers vs. 0 triggers ICP of direct  triggers vs. theories trig Calculations for 7 < pT < 9 GeV/c STAR Preliminary STAR Preliminary • Icp agrees with theoretical predictions. • Within the current uncertainty in the scaling the Icp of direct  and 0 are similar. A. Hamed STAR Experiment HP2008 Spain 8-14th June

  19. Luminosity Projections -jet yield Away-side hadrons  ET > 15 GeV PRL 98 (2007) 212301 Projection for statistical uncertainties in γ-hadron suppression as the integrated luminosity increases. Projection is for ETγ> 15 GeV, associated particle pT from 4-6 GeV/c. Phys. Rev. C74 (2006) 034906 More precision is required to nail down the medium density A. Hamed STAR Experiment HP2008 Spain 8-14th June

  20. Summary and Outlook Summary and Outlook • First result of -jet azimuthal correlations and fragmentation function D(zT) in AuAu at RHIC energy is reported. • Shower shape analysis is effective for /0 discrimination. • All results of 0’s near and away-side associated particle yields shows consistency with that of charged hadron triggers. • The IAA of -jet agrees with the theoretical predictions. • Within current systematic uncertainties, -triggered IAA is similar to RAA of inclusive hadrons and is similar to 0’s IAA. • Direct photon-hadron correlations are moving from the proof of principle stage towards the stage of precision measurement. A. Hamed STAR Experiment HP2008 Spain 8-14th june A. Hamed STAR Experiment HP2008 Spain 8-14th June

  21. Thank you for your attention and many thanks to all STAR collaborators

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