Prospects for understanding energy loss in hot nuclear matter Marco van Leeuwen, Utrecht University

Goals for hard probes In my opninion, we aim to 1. Understand energy loss and in-medium fragmentation and use it to 2. Measure the medium density (properties) in heavy ion collisions 1. is interesting in its own right as long as we can make contact with good QCD theory 2. is of obvious interest for heavy ion physics Two questions, so probably need two or more observables and a combined fit

What can we learn from RAA? p0 spectra Nuclear modification factor PHENIX, PRD 76, 051106, arXiv:0801.4020 Too-simple model favors DE/E constant Renk et al: favor DE constant Ball-park numbers: DE/E ≈ 0.2, or DE ≈ 2 GeV not small compared to 10-20 GeV parton at RHIC Note: slope of ‘input’ spectrum changes with pT: use experimental reach to exploit this

Energy loss distribution from theory Energy loss distribution P(DE) is where we can constrain/compare to QCD theory (BDMPS-Z, GLV, AMY) ‘Typical for RHIC’ TECHQM ‘brick problem’ L = 2 fm, DE/E = 0.2 E = 10 GeV Not a narrow distribution: Significant probability for DE ~ E Conceptually/theoretically difficult Significant probability to lose no energy (e.g. eikonal approximation not valid in principle) + geometry. Nuisance, or tool?

RAA at LHC GLV BDMPS T. Renk, QM2006 RHIC RHIC S. Wicks, W. Horowitz, QM2006 LHC: typical parton energy > typical E Expected rise of RAA with pT depends on energy loss formalism Nuclear modification factor RAA at LHC sensitive to radiation spectrum P(E) Independence of RAA on pT at RHIC due to fine-tuning/interplay of different trends?

Parton energy from g-jet and jet reconstruction second-generation measurements at RHIC Qualitatively: known pQCDxPDF extract `known’ from e+e- Full deconvolution large uncertainties (+ not transparent) Fix/measure Ejet to take one factor out  Two approaches: g-jet Jet reconstruction

Direct-g prospects at RHIC Expected recoil for various P(DE) Projected uncertainties (with stochastic cooling at RHIC) T. Renk ET,g Measurement sensitive to energy loss distribution P(DE) Need precision to distinguish scenarios Large luminosities expected at RHIC, precision increases But still ET,trig ~ ET,jet ~ 10 GeV 7

Rates for g-jet at RHIC and LHC g-jet rates B. Jacak, W. Vogelsang Need plot P. Jacobs, M. van Leeuwen g/p-ratio larger at RHIC than LHC With |h| < 1, reach 70-80 GeV for g-jet at LHC

Jets at LHC Simulated result (Jets at RHIC covered by Peter Jacobs) Simulated result ALICE EMCal TDR Jet fragmentation should be sensitive to energy loss model. e.g.ALICE jet quenching code with Ejet = 175 GeV However: Need more theoretical or theory/experiment exploration of capabilities Large hard process yields: Jets to > 200 GeV Light, heavy hadrons to 100 GeV

Are there other sensitive observables? One idea that is around: PTt1 pTa1 pTt1 pTt2 T.Renk,arXiv:0804.1204 2 density models Idea: use back-to-back hadron pair to trigger on di-jet and study assoc yield PTt2 Tune/control fragmentation bias and possibly geometry/energy loss bias Does this constraint P(DE) better

Associated yields from coalescence Importance of particle identification? If coalescence/recombination really plays a role, we should see: ‘Shower-thermal’ recombination Recombination of thermal (‘bulk’) partons Baryon pT=3pT,parton Meson pT=2pT,parton Baryon pT=3pT,parton Meson pT=2pT,parton and/or Hot matter Hot matter Hard parton (Hwa, Yang) No jet structure/associated yield Expect large baryon/meson ratio associated with high-pT trigger Expect reduced associated yield with baryon triggers 3<pT<4 GeV Current experimental results not conclusive (limited pT-range and large uncertainties) Worth pursuing!

Summary Energy loss distribution P(DE) is key to hard probes of QGP Integrating observables (single- and di-hadron suppression) only sensitive with large dynamic range (e.g. LHC) g-jet should be sensitive, statistics limited Jet fragmentation (spectra) should be sensitive Currently, limits of sensitivity etc not explicit Need theory-experiment work to clarify Can we identify other observables (e.g. 2+1 hadrons)?

Projected performance for g-hadron measurement Outlook II: RHIC Accelerator upgrades Stochastic cooling Detector upgrades Vertex detectors PHENIX Add rate plot? Projected performance for g-hadron measurement STAR Enables charm/bottom direct measurements Ongoing data analysis: Large sample Au+Au (run-7) and d+Au (run-8)

Direct-g recoil suppression  Expected recoil for various P(DE) T. Renk Measurement sensitive to energy loss distribution P(DE) Need precision to distinguish scenarios 8 < ET,g < 16 GeV 2 < pTassoc < 10 GeV J. Frantz, Hard Probes 2008 A. Hamed, Hard Probes 2008 STAR Preliminary ET,g DAA (zT) IAA(zT) = Dpp (zT) Large suppression for away-side: factor 3-5 Results agree with model predictions Uncertainties still sizable Some improvements expected for final results Future improvements with increased RHIC luminosity 14

Au+Au vs d+Au comparison T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c Au+Au d+Au Df -1 -2 1 2 3 4 5 1 _dN_ Ntrig d(Df ) STAR Preliminary 200 GeV Au+Au & d+Au T1 T.Renk,arXiv:0804.1204 2 density models O. Barannikova, F. Wang, QM08 T2 Au+Au similar to d+Au Model calculation: DE smallest when PTT1~PTT2 (still DE>0) Di-hadron trigger selects jet pairs with little or no energy loss in Au+Au To do: increase PTT1-PTT2

Energy loss in QCD matter radiated gluon propagating parton QCD bremsstrahlung (+ LPM coherence effects) m2 Transport coefficient l Energy loss Energy loss probes: Density of scattering centers: Nature of scattering centers, e.g. mass: radiative vs elastic loss Or no scattering centers, but fields  synchrotron radiation?