What’s Missing in our Current Picture from High p T Measurements at RHIC? Saskia Mioduszewski Texas A&M University 23 March, 2007

Single-Particle Spectra – What have we learned? 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)

What can we learn about Energy Loss? Fractional effective energy loss: S loss (MJT) “Effective” because of surface bias when analyzing single particle spectra PHENIX  0 Spectrum Renk and Eskola, hep-ph/ < p T < 15 GeV/c PHENIX, nucl-ex/

New Understanding of Energy Loss through Heavy Flavor? GLV calculation requires collisional energy loss to describe electrons from heavy-flavor decays  perhaps collisional energy loss not negligible as previously assumed STAR: nucl-ex/ , PHENIX (QM2006): nucl-ex/ S. Wicks, QM 2006

Heavy flavor suppression– Challenge to Theory Electrons from heavy-flavor decays are more suppressed than expected, in particular with non-zero contribution from bottom p+p collisions X. Lin, QM 2006

pedestal and flow subtracted 4 < p T,trig < 6 GeV/c, 2< p T,assoc < p T,trig Di-Jets through Hadron-Hadron Correlations “Disappearance of away-side jet” in central Au+Au collisions 0-5% Escaping Jet “Near Side” Lost Jet “Far Side” STAR, PRL 90 (2003) I AA  (Jet-correlated Yield in AA) / (Jet-correlated Yield in pp)

Evolution of Jet Structure At higher trigger p T (6 < p T,trig < 10 GeV/c), away-side yield varies with p T,assoc For lower p T,assoc ( 1.3 < p T,assoc <1.8 GeV/c), away-side correlation has non-gaussian shape  becomes doubly-peaked for lower p T,trig pedestal and flow subtracted 4 < p T,trig < 6 GeV/c, 2 < p T,assoc < p T,trig M. Horner, QM 2006

STAR preliminary 0-12% 200 GeV Au+Au Hard-soft correlations Hard-soft: away-side spectra approaching the bulk. Inclusive in top 5%?  Three-particle correlation – N.N. Ajitanand, J. Ulery Medium away near deflected jets away near Medium mach cone STAR, PRL 95, (2005) 4 < pT,trig< 6 GeV/c

Near-side Correlation Additional long-range correlation in  Au+Au 20-30% the “ridge” Coupling of high p T partons to longitudinal expansion - Armesto et al, PRL 93 (2004) QCD magnetic fields- Majumder et al, hep- ph/ In recombination framework: Coupling of shower partons to thermal partons undergoing longitudinal expansion- Chiu & Hwa Phys. Rev. C72:034903,2005 Radial flow + trigger bias – S.A. Voloshin, Nucl. Phys. A749, 287 (2005) J. Putschke, QM 2006 Au+Au 0-10% STAR preliminary

 (J+R) |  |<1.7 J = near-side jet-like corrl. R = “ridge”-like corrl. v 2 modulated bkg. subtracted  (J+R) |  |<1.7 flow (v 2 ) corrected Extracting near-side “jet-like” yields 1 Au+Au 20-30% 2 2  (J+R) -  (R) const bkg. subtracted  (J) |  |<0.7   (J) no bkg. subtraction const bkg. subtracted  (J) |  |<0.7 J. Putschke, QM 2006

STAR preliminary “Jet” spectrum vs. “Ridge” spectrum “jet” slope “ridge” slope inclusive slope J. Putschke, QM 2006 STAR preliminary

Ridge Yield p t,assoc. > 2 GeV STAR preliminary Ridge yield persists up to highest trigger p T and approximately constant yield J. Putschke, QM 2006

“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 < pT,trig< 15 GeV/c STAR, Phys. Rev. Lett. 97 (2006)

Comparison of I AA to R AA I AA = Yield(0-5% Au+Au) Yield(d+Au) In the di-jets where trigger p T is 8-15 GeV/c, the suppression is same as for single particles as a function of p T = Near-side I AA = Away-side I AA 8 < p T (trig) < 15 GeV/c D. Magestro, QM 2005

Near-side Yields vs. z T After subracting the RidgeM. Horner, QM 2006

Away-side Yields vs. z T M. Horner, QM 2006

Away-side suppression as a function of p T,trig M. Horner, QM 2006 Away-side I AA Away-side suppression reaches a value of 0.2 for trigger p T > 4 GeV/c, similar to single- particle suppression I AA  (Jet-correlated Yield in AA) / (Jet-correlated Yield in pp)

What other handles do we have? Centrality, trigger and associated p T,….. ….Reaction plane In-plane Out-plane STAR 4 < p T,trig < 6 GeV/c, 2 < p T,assoc < p T,trig STAR, Phys. Rev. Lett. 93 (2004)

Another handle:  -jet  q Photon-jet measurement is, in principle, sensitive to full medium Bias to where away-side jet is close to surface? Together with di-jet measurement for comparison  Another differential observable Increasing ratio of direct photons to decay photons with centrality due to hadron suppression at high p T PHENIX, Phys. Rev. Lett. 94, (2005) Wang et al., Phys.Rev.Lett. 77 (1996)

1/N trig dN/d   (rad) Another handle:  -jet Current Results from Run-4 Au+Au collisions: J. Jin, QM 2006 T. Dietel, QM 2005  q

Summary Limited information extracted from single-particle p T spectra –Effective fractional energy loss reaches 20% for most central collisions –Initial energy density ~ 15 GeV/fm 3 from radiative energy loss models Di-Jets (those that are observed) may have less surface bias Photon-Jet Measurement will complement the di-jet for more complete probe Heavy-flavor suppression not consistently described by theoretical models with light meson suppression – need elastic energy loss

Conclusions What is missing from our picture? Need a consistent theoretical approach to describe different observables Need more quantitative model predictions for “ridge” explanation Need more quantitative descriptions of jet modifications from experimenters –Particle species in ridge vs. in jet (J. Bielcikova’s talk) –Path-length dependence of jet-correlated yields (using reaction plane dependence) Need a great deal of statistics for  -jet measurement (J. Dunlop’s talk)

STAR preliminary Jet+Ridge (  ) Jet (  ) Jet  ) yield ,  ) N part 3 2 GeV

S loss S(pT )/pT = S0, is a constant for all pT > 3 GeV/c, which also results in a constant ratio of the spectra, RAA(pT ). For the constant fractional shift, the Jacobean is simply dS(pT )/dpT = S0 RAA(pT ) = (1 + S0) −n+2 RAA(pT ) 1/(n−2) =1/(1 + S0) The effective fractional energy loss, Sloss, is related to the fractional shift in the measured spectrum, S0. The hadrons that would have been produced in the reference p + p spectrum at transverse momentum pT + S(pT ) = (1 + S0)pT, were detected with transverse momentum, pT, implying a fractional energy loss: Sloss = 1 − 1/(1 + S0) = 1 − RAA(pT ) 1/(n−2)