1. 1 34th International Conference on High Energy Physics (ICHEP 2008) ‏ The STAR Experiment Texas A&M University A. Hamed for the STAR collaboration Direct  -charged hadron azimuthal correlation measurements Table of Contents:  Introduction  Results  Analysis  Summary

2. 2 A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th. Introduction: Degrees of freedom The space-time evolution of a relativistic heavy-ion collision  One of the most important characteristics is the medium color charge density, which might lead to understand the medium dynamics.  Nuclear density  0 ~ 0.15nucleons/fm 3  Specific volume ~ 6fm 3  Typical hadronic volume ~ 1-3 fm 3  The average inter- Nucleon distance in the nucleus ~ 1.8fm  One must expect in case of nuclear density greater than 3  0 the nucleons to overlap, and their individuality to be lost. J.C. Collins, M.J. Perry, Phys. Rev. Lett. 34 1353(1975). Ordinary nuclear matter How to probe the color charge density? Small size “ a few fermi in diameter” Very short life time 5-10x10 -23 s.

3. 3 Central Au+Au Gluon radiation is induced by multiple scattering A particle distribution in fractional energy is softened in the medium A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th. Introduction: Hard probes- I  Like QED the charge density of the medium can be probed by its effect on the propagation of a fast particle.  Hard processes  Take place at early time of collisions a good probe of the medium. How the hard probes can be used to measure the modification on the FF? p+p or peripheral Au+Au Hard Scattering in vacuum-QCD Hard Scattering in the medium Compareversus . D med (z, P(  E,E)) ‏ h/a Fragmentation Function Study the particles distribution in fractional energy. D vac h/a (z) ‏ Very short life time medium

4. 4 A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th. Introduction: Hard probes - II Jet-like azimuthal correlations “conservation of linear momentum” p+p  di-jet Trigger  An access to the parton initial energy is required to quantify the energy lost Associated particles Near side  0 Away side  Trigger Associated particles  Au+Au  ?  In the near-side p+p, d+Au, and Au+Au are similar while in the away-side “back-to-back” Au+Au is strongly suppressed relative to p+p and d+Au. 4 < p T,trig < 6 GeV/c 2 < p T,assoc < p T,trig PRL. 91, 072304 (2003)‏ Background is subtracted Central Au+Au ?  E parton E trigger hadron How much energy is lost in here? conservation of linear momentum

5. 5 Introduction: Jet-energy calibration “Direct  ” A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th. 0  “Mid-rapidity” P P Fast Detector “Calorimeter” Leading particle “trigger” xP Associated particles Background How much energy is lost in the medium? FF is softened in the medium No access to the parton initial energy Color charge density? get the initial parton energy with a powerful alternative method: “Direct  -hadron azimuthal correlations” How to measure direct  -hadron azimuthal correlations? Due to fragmentation full jet reconstruction is required to access the initial parton energy OR zero near-side yield for direct photons Direct photons escape from the medium without any further interactions

6. 6 Correlate photon candidate “triggers” with “associated tracks”  Use  triggers to explore fragmentation functions in p+p and Au+Au 00 2 E π ‹ E parton 0 BEMC Beam axis TPC Analysis: Analysis technique A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th. p T,trig > 8 GeV/c 180° E γ = E parton Associated charged particles “3 <p T,assoc < 8 GeV/c” How to distinguish between  0 /  ? BEMC: Barrel Electro-Magnetic Calorimeter TPC: Time Projection Chamber Full azimuthal coverage No track with p > 3 GeV/c points to the trigger tower One tower out of 4800 towers (0.05 x 0.05) ‏ ~2.2m Charged hadrons

7. 7 The two photons originated from  0 hit the same tower at p T >8GeV/c Analysis: Shower Shape Analysis A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th.  i : strip energy r i : distance relative to energy maxima  7 R M 00 Use the shower-shape analysis to separate the two close photons shower from one photon shower. STAR Shower Maximum Detector is embedded at ~ 5x 0 between the lead-scintillator layers “BEMC”

8. 8 Results: Effect of shower-shape cut A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th. o The away-side correlation strength is suppressed compared to pp and peripheral Au+Au. Medium effect o The  -rich sample has lower near-side yield than  0 but not zero.  -sample is not pure direct  ! How about the  0 ? Vacuum QCD Centrality Background is not subtracted

9. 9 A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th. Results: Comparison of  0 -triggered yields to charged-hadron triggered yields Completely different data set from different RHIC runs, different detectors were involved in the analysis too. Associated yields per trigger  0 -charged and charged-charged results are consistent. Near side: Yields are similar for p+p and central Au+Au Central Au+Au ? Surface bias  0 sample is pure. PRL 97 162301 (2006). This analysis Away side: Yields show big difference between p+p and central Au+Au

10. 10 0  Extraction of direct  away-side yields R=Y  -rich+h /Y  0+h near Y  +h = (Y  -rich+h - RY  0+h )/(1-R) ‏ away Assume no near-side yield for direct  then the away-side yields per trigger obey A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th. Results: Method of extract direct  associated yield This procedure removes correlations due to contamination (asymmetric decay photons+fragmentation photons) with assumption that correlation is similar to  0 – triggered correlation at the same p T. O(α s 2 α(1/α s +g))‏

11. 11 Direct  00 Associated yields per trigger A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th. Results: Fragmentation function of direct  triggers and  0 triggers The away-side yield per trigger of direct  triggers shows smaller value compared to  0 triggers which is consistent with partons loose energy “dense medium” and then fragment. Differences between  and  0 triggers  0 -triggers are resulted from higher parton energy than  -triggers.  0 -triggers are surface biased. Color factor effect. What is the medium color charge density?

12. 12  I cp agrees with theoretical predictions. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th. Results: Medium effect on fragmentation function I cp (z T ) = D 0-10% (z T ) ‏ D 40-80% (z T ) ‏ STAR Preliminary 7 < p T < 9 GeV/c trig More precision is needed for the measurements to distinguish between different color charge densities. STAR Preliminary Within the current uncertainty in the scaling the I cp of direct  and  0 are similar. If there is no medium effect I cp (z T ) = 1 Strong medium effect I AA (z T ) = D AA (z T ) ‏ D pp (z T ) ‏ 8 < p T < 16 GeV/c trig p T > 3 GeV/c assoc Data points

13. 13 First result of  -jet azimuthal correlations and fragmentation function D(z T ) in AuAu at RHIC energy is reported. All results of  0 ’s near and away-side associated particle yields shows consistency with that of charged hadron triggers. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th. Summary and Outlook Large luminosity at RHIC enables these measurements. Expect reduced uncertainties from further analysis and future runs. Away-side yield for direct photons is significantly suppressed in heavy ion events. Suppression level agrees with theoretical expectations.

14. 14 Thank you for your attention and many thanks to all STAR collaborators

15. 15 Backup slides

16. 16 Shower Shape Cuts: Reject most of the  0 ’s. highly asymmetric  0 decay. But do not reject photons from:  ’s - similar level of background as asymmetric  0 fragmentation photons  10% of all  0 with p T > 8 GeV/c  10% of inclusive  at intermediate p T in p+p ~30-40% of direct  at P T > 8 GeV/c. Limitations of the shower shape cut A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th.

17. 17  -jet yield Away-side hadrons Phys. Rev. C74 (2006) 034906 E T > 15 GeV  More precision is required to nail down the medium density PRL 98 (2007) 212301 Projection for statistical uncertainties in γ-hadron suppression as the integrated luminosity increases. Projection is for E T γ> 15 GeV, associated particle p T from 4-6 GeV/c. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th. Luminosity Projections

18. 18 00  7 R M  Two photons (  0 ) produce a more diffuse shower than single photons (  )‏ ∑ i  i r i 1.5 E total  Wider shower has small value of such quantity  i : strip energy r i : distance relative to energy maxima Two dimensional shower shape Very pure sample of  0 Shower shape cut for  0  selection is not tight   rich sample Shower shape cut for  On the transverse shower profile cut A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th.

19. 19 Away-side Y  -rich+h =1/N  -rich (N  -rich+h ) ‏ =1/N  -rich [N  0+h +N  +h ] =(N  0+h /N  -rich )+(N  +h /N  -rich ) ‏ Y  +h =(N  -rich /N  )[Y  -rich+h -(N  0 /N  -rich )*Y  0+h ] =(N  0 /N  -rich )*N  0+h /N  0 +(N  /N  -rich )*N  +h /N  Y  0+h Y  +h 1 Near-side Y  -rich+h =(1/N  -rich )*N  -rich+h =(1/N  -rich )*N  0+h N  +h =0 =(N  0 /N  -rich )*Y  0+h Solve for Y  +h 3 unknowns N  -rich, N  0, and N . N  0 /N  -rich =Y  -rich+h /Y  0+h =R Y  +h =(N  -rich /N  )[Y  -rich+h -R*Y  0+h ] 1-R=1-(N  0 /N  -rich )=(N  -rich -N  0 )/N  -rich =N  /N  -rich 1/(1-R)=N  -rich /N  Substitute in 1 2 Substitute in 2 Y  +h =[Y  -rich+h -R*Y  0+h ]/1-R 3 (N  -rich -N  0 )=N  A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29 th -August 5 th. Method of extract direct  associated yield