Hard Probes at RHIC Saskia Mioduszewski Texas A&M University Winter Workshop on Nuclear Dynamics 8 April, 2008.
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Hard Probes at RHIC Saskia Mioduszewski Texas A&M University Winter Workshop on Nuclear Dynamics 8 April, 2008
Outline Review of High p T Probes Motivation for -jet Measurement Experimental Methods Current Status of Measurement
Single-Particle Spectra Hadrons are suppressed, photons are not – photons serve as the “control” experiment High-p T suppression comparison to theory: GLV dN g /dy ~ 1000 I. Vitev and M. Gyulassy, PRL 89 (2002) 252301 Initial energy density: 0 ~ 15 GeV/fm 3 PHENIX, Phys. Rev. Lett. 96, 202301 (2006)
From Single-Particle Spectra to 2-Particle Correlations Single inclusive hadronsDi-jets (hadron-hadron correlations) Suppression quantified by R AA = Yield AA /(Yield pp * pp ) Single particle is leading hadron of jet Probe of density of medium, but Strong surface bias for “trigger” particle Suppression quantified by I AA = Jet-assoc. Yield AA /Jet-assoc. Yield pp Hadrons associated with high-p T trigger particle More differential probe of energy loss Surface bias reduced, but not removed Trigger
Trigger Biases Escaping Jet “Near Side” Lost Jet “Far Side” q Photon trigger is sensitive to full medium True measure of the Energy (no energy loss for direct photon) Renk and Eskola, hep-ph/0610059 Single inclusive hadrons Di-jets (hadron-hadron correlations) Trigger particle
“Reappearance of away-side jet” With increasing trigger p T, away-side jet correlation reappears 4 < p T,trig < 6 GeV/c, 2< p T,assoc < p T,trig STAR, Phys. Rev. Lett. 97 (2006) 162301
Surface Bias of Di-Jets? Renk and Eskola, hep-ph/0610059 8 < pT,trig< 15, 4< pT,assoc< 6 GeV/c 8 < p T,trig < 15 GeV/c STAR, Phys. Rev. Lett. 97 (2006) 162301 Higher p T Jet “Near Side” “Punch-through” Jet? Tangential emission?
8 -Jet: “Golden Probe” of Energy Loss emerges unscathed from the medium - This probe is valuable for comparison with di-hadron correlations - Modification as a function of “true” parton energy real fragmentation function D(z) h q g q QCD analog of Compton Scattering Wang et al., Phys.Rev.Lett. 77 (1996) 231-234
Challenging Measurement Trigger photon measured with electro- magnetic calorimeter Large background from hadronic decays ( 0, ) Background sources of photons (hadronic decays) suppressed in central Au+Au collisions PHENIX, Phys. Rev. Lett. 94, 232301 (2005)
-jet Analysis in Au+Au collisions Thomas Dietel, Quark Matter 2005
Challenging Measurement Trigger photon measured with electro- magnetic calorimeter Large background from hadronic decays ( 0, ) Background signal is di-jet ( 0 -jet) Subtract from true signal Need large-statistics data set for -jet measurement
Statistical measurement of -jet yields Measure inclusive -jet correlation function Measure 0 -jet correlation function Construct resulting decay -jet correlation taking into account decay kinematics Subtract decay -jet from inclusive -jet Result is direct -jet
PHENIX -jet in p+p PHENIX Preliminary J.Jin, QM 2006 Comparison to PYTHIA – verification of method # inclusive # decay R =
Away-side integrated yields trigger p T dependence assoc. h p T dependence centrality dependence + pp p+p shows a systematic trend of having higher yields in the away side than Au+Au! J. Jin, QM 2006
Cu+Cu direct -h +/- vs. -h +/- jet yields per-trigger jet pair yields: systematic from R systematic from subtraction method A. Adare, WWND 2007
Distinguishing direct photons from 0 p T (GeV/c) Opening angle (radians) Opening angle of 0 as a function of p T Segmentation of calorimeter towers in STAR ( ) = (0.05,0.05) 2 decay photons hit same tower starting p T ~ 5 or 6 GeV/c Use SMD to discriminate Segmentation in PHENIX ~ 0.01 radians
Distinguishing direct photons from 0 Reject clusters that look like 2 close photons (from 0 decay) from shower shape Retain sample of correlations that are rich -jet Estimate remaining contamination from 0 - jet from near-side yields Measure and subtract 0 -jet away-side yield
Backgrounds Asymmetric 0 decays (~10% of 0 for p T >8 GeV/c) decays (~5% contribution in central Au+Au collision, as much as 25% in p+p) Fragmentation photons – subtracted to the extent that correlations are similar to 0 - triggered correlations
Distinguishing single photons from 2 decay photons in STAR Shower-max detector ( )~(0.007,0.007)
21 Comparison of Near-side yields ( 0 -charged versus charged-charged PRL 97(2006)162301 ),assoc Similar near-side yields for p+p, d+Au and Au+Au Agreement between charged-charged and 0 -charged correlations Important cross-check that shower-shape PID for 0 is good!
22 Comparison of away-side yields,assoc Smaller away- side yield in Au+Au compared to pp and d+Au.
23 I AA of 0 I AA of the away-side in 0 -trigger shows similar suppression for all p T associated bins (3-8 GeV/c).
24 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 is calculated as
25 Direct- Away-Side Yields The away-side yield of the associated particle per trigger in -jet is suppressed.
Suppression similar level to inclusives in central collisions First measure of away-side I AA for -h T. Renk and K. Eskola PRC75:054910,2007 Good agreement between theory and measurement E jet = E = E trig A. Hamed, QM2008
27 Away-side Yields Relative to Peripheral Au+Au I cp of -jet exhibits same suppression on the away-side yield per trigger of the associated particles (3-8GeV/c). A. Hamed, QM2008 Peripheral Au+Au ~ p+p = vacuum
Summary Correlation measurements provide more differential probe of energy loss than single-particle p T spectra Di-Jets (those that are observed) may have less surface bias Photon-Jet Measurement will complement the di-jet for more complete probe
Summary Both STAR and PHENIX have measured a correlation function for direct +jet Preliminary associated-hadron yields have been measured Large statistics necessary Next step is to show fragmentation function in p+p vs. Au+Au