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)  Initial energy density:  0 ~ 15 GeV/fm 3 PHENIX, Phys. Rev. Lett. 96, (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/ 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)

Surface Bias of Di-Jets? Renk and Eskola, hep-ph/ < pT,trig< 15, 4< pT,assoc< 6 GeV/c 8 < p T,trig < 15 GeV/c STAR, Phys. Rev. Lett. 97 (2006) 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)

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, (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 =

PHENIX  -jet in Au+Au collisions J.Jin, QM 2006 Black: Au+Au Red: p+p 1/N trig dN/d   (rad)

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) ),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