The Art Poskanzer School 1

2 Physics motivation To create and study QGP – a state of deconfined, thermalized quarks and gluons predicted by QCD at high energy density. Lattice QCD prediction: T C ~ 170  8 MeV  C ~ 0.5 GeV/fm 3 Penetrating probe: High p T and jets Can be calculated from pQCD thus are well calibrated in vacuum. Are expected to be modified in nuclear medium hence provide information on medium properties. F. Karsch, hep-lat/

The Art Poskanzer School 3 Jet quenching – final state effect STAR, PRL 91, (2003). Vitev, Gyulassy, PRL 89, (2002); X.-N. Wang, PLB 595 (2004) 165. pQCD model calculations of partonic energy loss: x30 gluon density, x100 energy density in central Au+Au,  =10-20 GeV/fm 3, well above the predicted critical  C. “surface” emission strong absorption d+Au ~ pp. near side Au+Au ~ pp. away side Au+Au ~ 0.

The Art Poskanzer School 4 But… Baryon/meson puzzle! PHENIX, PRL 91, (2003) QM’04 Identified jet correlations! PHENIX – nucl-ex/ STAR – preliminary results Fries et al, PRC (2003); Greco et al, PRC 68, (2003); Hwa et al, PRC 70, (2004). QM 04

The Art Poskanzer School 5 State of Affair Experimental evidence of jet quenching well established. But baryon/meson puzzle! Key questions to ask:  Where does the energy go?  Amount of energy loss? medium contribution?  How is energy distributed? thermalization?

The Art Poskanzer School 6 Where did the energy go? need to measure the entire jet including low p T. p + p  jet + jet Au + Au  stuff + jet + jet e + + e    jet + jet measure those…in this?! High p T particle We can do it but with some care… angular correlation with a leading hadron – combinatorial background from mixed-events.

The Art Poskanzer School 7 Reconstructing low p T associated particles High p T particle p+p High p T particle Au+Au Take trigger particle. Form  and  correlations of other hadrons with trigger. Background from mix-events. Add v 2 modulation. Normalize in 0.8<|  |<1.2. Efficiency correction on associated particles. Take difference and normalize per trigger.  (1/N trig ) dN/d(  ) background Signal p+p Jet-like structures Au+Au top 5% Systematic uncertainties: background normalization and v 2 correction.

The Art Poskanzer School 8 Correlation functions STAR, nucl-ex/ Enhanced and broadened distribution at low p T. Away side suppression at high p T.

The Art Poskanzer School 9 Re-appearing at low p T M.G. Albrow et al. NPB145, 305 (1978). High p T Low p T Definition: near side: | Df | 1, | h |<1 Separation at  1 radian.

The Art Poskanzer School 10 Associated multiplicity and “energy” Same p T final trigger particle appears to select larger energy jet in central AA than in pp. p+p Is this the amount of energy loss?

The Art Poskanzer School 11 Jet quenching model X.-N. Wang, PLB 579 (2004) 299, nucl-th/ with energy loss without energy loss D E = 1.4 – 2.2 GeV }

The Art Poskanzer School 12 Vector momentum balance? TPC acceptance of away side partner. Total scalar p T : Initial parton energy + medium contribution Medium contribution to the away side associated energy? Leading hadrons Medium

The Art Poskanzer School 13 Medium contribution to associated energy? Shower-thermal recombination: Hwa, Yang, PRC 70, (2004): TS largest contribution to high p T particles. Mach shock waves: Stoecker, nucl-th/ Casalderra, Shuryak, Teaney, hep-ph/ Excess of energy on the away side most prominent at  1. How does medium energy become correlated with the trigger?

The Art Poskanzer School 14 Preliminary Correlations vs centrality Preliminary

The Art Poskanzer School 15 Correlations vs p T Preliminary RMS size Preliminary

The Art Poskanzer School 16 Shapes varies with p T Shape varies with p T, becomes broader and appears double peaked at large p T assoc. Effect is more pronounced for lower p T trig ! Vitev, hep-ph/ : should not have peak structure. 4 < p T trig < 6 GeV/c2.5 < p T trig < 3 GeV/c 0.3<p T assoc <0.8 GeV/c 0.8<p T assoc <1.3 GeV/c 1.3<p T assoc <1.8 GeV/c Preliminary

The Art Poskanzer School 17 syst. error Associated particles p T distributions Near side: overall enhancement from pp to AA larger initial parton energy (and modest energy loss)? Away side: softening of spectra from pp to central Au+Au energy from initial parton converted to low p T particles. energy loss in medium! 4 < p T trig < 6 GeV/c

The Art Poskanzer School 18 Away side Two sources of particles: hard: jet fragmentation products. soft: bulk medium decay products. Peripheral: very different Central: not much different Gradual decrease with centrality. Similar for two trigger p T windows. Whatever interaction mechanisms, away-side jet products approach equilibrium with the bulk medium traversed  thermalization of the bulk itself quite plausible. from away jets from medium decay Leading hadrons Medium

The Art Poskanzer School 19 Novel behavior of away Preliminary more robust than correlation functions. Novel dip structure observed in central AA. Energy loss effect? Mach shock wave? Leading hadrons Medium

The Art Poskanzer School 20 Away vs centrality Away core drops with centrality faster than corona. Core hadrons almost identical to medium in central collisions. Energy loss pattern: path-length effect? Mach shock wave manifestation? Preliminary

The Art Poskanzer School 21 Mach cone and/or deflected jets Deflected jets Mach cone

The Art Poskanzer School 22 Summary and open questions ? Statistical reconstruction of jets in pp, dA and AA. Connection between high p T and low p T physics. Same trigger p T appears to select larger energy jets in AA than in pp. Near side associated multiplicity is enhanced. Away side correlation disappears at high p T, and reappears at low p T. Correlations functions broadened. Interplay between energy loss and Mach shock wave? Significant softening of spectra from pp to central AA. Partial thermalization between jets and bulk medium. Imply high degree of thermalization in medium itself. To create and study QGP – a state of deconfined thermalized quarks and gluons predicted by QCD at high enerdy density

The Art Poskanzer School Backup slides--- Backup Slides

The Art Poskanzer School 24 Measuring the lost energy? possible… by going to low p T. Pal, Pratt, PLB 574 (2003) 21  How is energy distributed?  amount of energy loss?  contribution from medium? S. Pal, S. Pratt, PLB574 (2003) 21. X.-N. Wang, PLB 579 (2004) 299, nucl-th/ C.A. Salgado, U.A. Wiedemann, hep-ph/ M. Gyulassy, I. Vitev, X.-N. Wang, B.-W. Zhang, nucl-th/ ……

The Art Poskanzer School 25 Jet quenching model X.-N. Wang, PLB 579 (2004) 299, nucl-th/ with energy loss without energy loss D E = 1.4 – 2.2 GeV } Caution: cannot be readily compared to data yet.

The Art Poskanzer School 26 Thermalization at work? yes… by putting two sources of particles together: one from jet fragmentation that are initially hard. the other from bulk medium that are soft. going to low p T. medium jet

The Art Poskanzer School 27 Existing pp, pA results G. Boca et al. ZPC49, 543 (1991)M.G. Albrow et al. NPB145, 305 (1978) B. Alper, NPB 87 (1975) 41 ISR

The Art Poskanzer School 28 I AA – new vs old I AA Centrality dN/d  Old New Line = new with old v2.