Slide #1.

The STAR Experiment Direct charged hadron azimuthal correlation Table measurements of Contents: Introduction Analysis Results Summary 34th International Conference on High Energy Physics (ICHEP 2008) Texas A&M University A. Hamed for the STAR 1
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Slide #2.

Introduction: Degrees of freedom The space-time evolution of a relativistic heavy-ion collision Ordinary nuclear matter Nuclear density 0 ~ 0.15nucleons/fm3 Specific volume  ~ 6fm3 Small size “ Very short a few fermi in life time diameter” -23 5-10x10 s. Typical hadronic volume ~ 1-3 fm3 The average interNucleon distance in the nucleus ~ 1.8fm  One must expect in case of nuclear density greater than 30 the nucleons to overlap, nd their individuality to be lost. J.C. Collins, M.J. Perry, Phys. Rev. Lett. 34 1353(1975) One of the most important characteristics is the medium color charge density, which might lead to understand the medium dynamics. How to probe the color charge density? th -August 5th th. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th 2
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Slide #3.

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 Very short life time Take place at early time of collisions medium. p+p or peripheral Au+Au h/ z) a Hard Scattering in the medium versus ( E ,E )) vacuum-QCD D v(ac h/ z , a P Compare Study the particles distribution in D (med Fragmentation Function fractionalHard energy. Scattering in a good probe of the Central Au+Au Gluon radiation is induced by multiple scattering A particle distribution in fractional energy is softened in the medium .  How the hard probes can be used to measure the modification on the FF? 3 th -August 5th th. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th
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Slide #4.

Introduction: Hard probes - II Jet-like azimuthal correlations conservation “conservationofoflinear linearmomentum momentum” E  Eparton trigger hadron PRL. 91, 072304 (2003) Central Au+Au How much energy is lost in here? ? Trigger Trigger Near side Associated 0 particles Associate 4 < pT,trig < 6 GeV/c d 2 < pT,assoc < pT,trigparticles   Away side  Background is subtracted p+p di-jet 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 tothe p+penergy and lo n access to the parton initial energy issuppressed required torelative quantify d+Au. 4 th -August 5th th. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th
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Slide #5.

Introduction: Jet-energy calibration “Direct ” “Mid-rapidity” zero near-side yield for direct photons Fast Detector “Calorimeter” Leading particle “trigger” Background 0 P  Color charge density? How much energy is lost in the medium? Due to fragmentation full jet reconstruction is required to access the initial parton energy OR xP xP Direct photons No access to the escape fromenergy the parton initial medium without any further FF is interactions softened P in the medium Associated particles get the initial parton energy with a powerful alternative method: “Direct -hadron azimuthal correlations” How to measure direct -hadron azimuthal correlations? th -August 5th th. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th 5
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Slide #6.

Analysis: Analysis technique Correlate photon candidate “triggers” with “associated tracks” pT,trig > 8 GeV/c Charged hadrons BEMC TPC One tower out of 4800 towers (0.05 x 0.05) .2 ~2 m 2 0 Eπ 0‹ No track with p > 3 GeV/c points to the trigger tower Ep 180° Use  triggers to explore fragmentation function in p+p and Au+Au Eγ = Eparton am e B is ax arto n BEMC: Barrel Electro-Magnetic Calorimeter TPC: Time Projection Chamber Associated charged particles “3
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Slide #7.

Analysis: Shower Shape Analysis The two photons originated from 0 hit the same tower at pT>8GeV/c STAR Shower Maximum Detector is embedded at ~ 5x0 between the lead-scintillator layers “BEMC” i : strip energy ri : distance relative to energy maxima  7 RM 0 Use the shower-shape analysis to separate the two close photons shower from one photon shower. 7 th -August 5th th. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th
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Slide #8.

Results: Effect of shower-shape cut Centrality Centrality Background is not subtracted Vacuum QCD Medium effect oThe away-side correlation strength is suppressed compared to pp and peripheral Au+Au. oThe -rich sample has lower near-side yield than 0 but not zero. -sample is not pure direct  ! How about the 0 ? th -August 5th th. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th 8
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Slide #9.

sults: Comparison of 0-triggered yields to charged-hadron triggered yie Completely different data set from different RHIC runs, different detectors were involved in the analysis too. Associated yields per trigger This analysis Surface bias PRL 97 162301 (2006). ? Central Au+Au Near side: Yields are similar for p+p and Away side: Yields show central big Au+Au difference between p+p and central Au+Au 0-charged and charged-charged results are consistent. 0 sample is pure. 9 th -August 5th th. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th
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Slide #10.

Results: Method of extract direct  associated yield O(αs2α(1/αs+g)) Extraction of direct away-side yields near near R=Y-rich+h/Y0+h Assume no near-side yield for direct   then the away-side yields per trigger obey away away 0  Y+h = (Y-rich+h - RY0+h )/(1-R) This procedure removes correlations due to contamination (asymmetric ay photons+fragmentation photons) with assumption that correlation is similar 0 – triggered correlation at the same pT. 10 th -August 5th th. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th
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Slide #11.

Associated yields per trigger Results: Fragmentation function of direct  triggers and 0 triggers Direct  0 Differences between  and 0 triggers 0 -triggers are resulted from higher parton energy than -triggers. 0 -triggers are surface biased. Color factor effect. away-side yield per trigger of direct  triggers shows smaller va compared to 0 triggers which is consistent with partons loose energy “dense medium” and then fragment. What is the medium color charge density? th -August 5th th. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th 11
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Slide #12.

Results: Medium effect on fragmentation function Icp(zT) = 1 D (z ) Icp(zT) = 0-10% T D40-80% (zT) D (z ) IAA(zT) = AA T Dpp (zT) Data points trig 8 < pT < 16 GeV/c assoc If there is no medium effect pT > 3 GeV/c Strong medium effect trig 7 < pT < 9 GeV/c STAR STAR Preliminary Preliminary  Icp agrees with theoretical predictions. More precision is needed for the measurements to distinguish between different charge densities. Withincolor the current uncertainty in the scaling the Icp of direct  and 0 are similar. th -August 5th th. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th 12
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Slide #13.

Summary and Outlook First result of -jet azimuthal correlations and fragmentation function D(zT) in AuAu at RHIC energy is reported. Away-side yield for direct photons is significantly suppressed in heavy ion events. Suppression level agrees with theoretical expectations. All results of 0’s near and away-side associated particle yields shows consistency with that of charged hadron triggers. Large luminosity at RHIC enables these measurements. Expect reduced uncertainties from further analysis and future runs. 13 th -August 5th th. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th
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Slide #14.

Thank you for your attention and many thanks to all STAR collaborators 14
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Slide #15.

Backup slides 15
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Slide #16.

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 0 fragmentation photons 10% of inclusive  at intermediate pT in p+p ~30-40% of direct  at PT > 8 GeV/c. 16 th -August 5th th. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th
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Slide #17.

Luminosity Projections -jet yield Away-side hadrons  ET > 15 GeV PRL 98 (2007) 212301 Phys. Rev. C74 (2006) 034906 Projection for statistical uncertainties in γ-hadron suppression as the integrated luminosity increases. Projection is for ET γ> 15 GeV, associated particle pT from 4More precision is required to nail down the medium density 6 GeV/c. 17 th -August 5th th. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th
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Slide #18.

On the transverse shower profile cut EtotalTwo photons (0) produce a more diffuse shower than single photons ( ∑i i ri1.5 Wider shower has small value of such quantity  7 RM 0 Two dimensional shower shape i : strip energy Shower shapeShower shape cut for 0 cut for  Very pure sample of 0 ri : distance relative to energy maxima  selection is not tight  rich sample 18 th -August 5th th. A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th
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Slide #19.

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