T Hallman SC MTG Jan 200 Evidence for

T. Hallman SC MTG Jan 200 Evidence for the Production of the Quark-Gluon Plasma at RHIC Tim Hallman Scientific Council Meeting Dubna, Russia January 20 -21, 2005 1

T. Hallman SC MTG Jan 200 A Definition of the Quark-Gluon Plasma QGP a (locally) thermally equilibrated state of matter in which quarks and gluons are deconfined from hadrons, so that color degrees of freedom become manifest over nuclear, rather than merely nucleonic, volumes. Not required: Ø non-interacting quarks and gluons Ø 1 st- or 2 nd-order phase transition Ø evidence of chiral symmetry restoration This definition is consistent within the community and ov 2

T. Hallman SC MTG Jan 200 Elliptic Flow at RHIC Anisotropic (Elliptic) Transverse Flow Peripheral Collisions • The overlap region in peripheral collisions is not sy – Almond shaped overlap region • Easier for particles to emerge in the direction of x-z plane • Larger area shines to the side – Spatial anisotropy Momentum anisotropy • Interactions among constituents generates a pressure gradient which transforms the i • Perform a Fourier decomposition of the momentum z y x Anisotropic Flow py • v 2 is the 2 nd harmonic Fourier coefficient of px 3

T. Hallman SC MTG Jan 200 Soft Sector: Evidence for Thermalization and EOS with Soft Point? Hydro calculations: Kolb, Heinz and Huovinen § Systematic m-dependence of v 2(p. T) suggests common transverse vel. field § m. T spectra and v 2 systematics for mid-central collisions at low p. T are well (~20 -30% level) described by hydro expansion of ideal relativistic fluid § Hydro success suggests early thermalization, very short mean free path § Best agreement with v 2 and spectra for therm < 1 fm/c and soft (mixed-phasedominated) EOS ~ consistent with LQCD expectations for QGP hadron 4

T. Hallman SC MTG Jan 200 How Unique & Robust is Hydro Account in Detail? Ø Are we sure that observed v 2 doesn’t result alternatively from harder EOS (no transition) and late thermalization? Ø How does sensitivity to EOS in hydro calcs. compare quantitatively to sensitivity to other unknown features: e. g. , freezeout treatment (compare figures at right), thermaliz’n time, longitudinal boost non-invariance, viscosity? Ø What has to be changed to understand HBT (below), and what effect will that change have on soft EOS conclusion? Sharp freezeout dip P. Kolb, J. Sollfrank, and U. Heinz, Phys. Rev. C. C 62 054909 (2000). Hydro+RQMD no dip? Hydro vs. STAR HBT Rout/Rside Teaney, Lauret & Shuryak 5

T. Hallman SC MTG Jan 200 Self-Analyzing (High p. T) Probes of the Matter at RHIC Nuclear Modification Factor: nucleon-nucleon cross section <Nbinary>/sinelp+p AA leading particle suppressed hadrons q q ? If R = 1 here, nothing new going on 6

T. Hallman SC MTG Jan 200 Hard Sector: Evidence for Parton Energy Loss in High Density Matter PHENIX § Inclusive hadron and away-side correlation suppression in central Au+Au, but not in d+Au, clearly establish jet quenching as final-state phenomenon, indicating very strong interactions of hard-scattered partons or their fragments with dense, dissipative medium produced in central Au+Au. 7

T. Hallman SC MTG Jan 200 Questions for Parton Energy Loss Models § p. QCD parton energy loss fits to observed central suppression d. Ngluon/dy ~ 1000 at start of rapid expansion, i. e. , ~50 times cold nuclear matter gluon density. § ~p. T-independence of measured RCP unlikely that hadron absorption dominates jet quenching. §How sensitive is this quantitative conclusion to: assumptions of factorization in-medium and vacuum fragmentation following degradation; treatments of expansion and initial-state cold energy loss preceding hard collision? § Can p. QCD models account for orientation - dependence of di-hadron correlation? Should be sensitive to both path length and matter expansion rate variation with ( R). 8

T. Hallman Soft Sector: Hadron Yield Ratios SC MTG Jan 200 STAR PHENIX Strangeness Enhancement Resonances § p. T-integrated yield ratios in central Au+Au collisions consistent with Grand Canonical stat. distribution @ Tch = (160 ± 10) Me. V, B 25 Me. V, across u, d and s sectors. § Inferred Tch consistent with Tcrit (LQCD) T 0 >Tcrit. § Does result point to thermodynamic and chemical equilibration, and not just phase-space dominance? 9

T. Hallman SC MTG Jan 200 Intermediate p T: Hints of Relevant Degrees of Freedom § For 1. 5 < p. T <6 Ge. V/c, see clear meson vs. baryon (rather than mass-dependent) differences in central-to-midcentral yields and v 2. § v 2/nq vs. p. T /nq suggestive of constituent-quark scaling. If better established exp’tally, would give direct evidence of degrees of freedom relevant at hadronization, and suggest collective flow @ constituent quark level. § N. B. Constituent quarks partons! Constituent quark flow does not prove QGP 10

T. Hallman Questions for Coalescence Models SC MTG Jan 200 Duke-model recomb. calcs. § Can one account simultaneously for spectra, v 2 and di-hadron correlations at intermediate p. T with mixture of quark recombination and fragmentation contributions? Do observed jet-like near-side correlations arise from small vacuum fragmentation component, or from “fast-slow” recombination? § Are thermal recomb. , “fast-slow” recomb. and vacuum fragmentation treatments compatible? Double-counting, mixing d. o. f. , etc. ? § Do coalescence models have predictive power? E. g. , can they predict centrality-dependences? 11

T. Hallman SC MTG Jan 200 Gluon Saturation: a QCD Scale for Initial Gluon Density + Early Thermaliz’n Mechanism? Saturation model curves use optical Glauber s. NN = 130 Ge. V Au+Au § Does the high initial gluon density inferred from parton E loss fits demand a deconfined initial state? Can QCD illuminate the initial conditions? § Assuming initial state dominated by g+g below the saturation scale (constrained by HERA e-p), Color Glass Condensate approaches ~account for RHIC bulk rapidity densities d. Ng/dy ~ consistent with parton E loss. § How robust is agreement, given optical vs. MC Glauber ambiguity in calcu -lating Npart , and assumption of ~one charged hadron per gluon? § CGC applies @ SPS too? If not, why is measured d. Nch/d ( s. NN) so smooth? 12

T. Hallman SC MTG Jan 200 Lattice QCD Predicts Some Sort of RAPID Transition! in entropy density, hence pressure The most realistic calcs. no discontinuities in thermodynamic proper-ties @ RHIC conditions (i. e. , no 1 st- or 2 nd-order phase transition), but still crossover transition with rapid evolution vs. temperature near Tc 160 – 170 Me. V. in chiral condensate in heavy-quark screening mass 13

T. Hallman SC MTG Jan 200 But What We Observe (at least in the soft sector) Appears Smooth : HBT parameters Charged particle pseudorapidity density p. T-integrated elliptic flow, scaled by initial spatial eccentricity No exp’tal smoking gun! Rely on theory-exp’t comparison Need critical evaluation of both! Theory must eventually explain the smooth energy- and centrality-dependences. 14

The Five Pillars of RHIC Wisdom. T. Hallman Ideal hydro SC MTG Jan 200 Early thermalization + soft EOS Statistical model Quark recombination constituent q d. o. f. …suggest appealing QGP-based picture of RHIC collision evolution, BUT invoke 5 distinct models, each with own ambiguities, to get there. p. QCD parton E loss u, d, s equilibration near Tcrit CGC Very high anticipated initial gluon density Very high inferred initial gluon density 15

T. Hallman SC MTG Jan 200 Summary on QGP Search All indications are that a qualitatively new form of matter is being produced in central Au. Au collisions at RHIC 1) The extended reach in energy density at RHIC appears to reach simplifying conditions in central collisions -- ~ideal fluid expansion; approx. local thermal equilibrium. 2) The Extended reach in p. T at RHIC gives probes for behavior inaccessible at lower energies – jet quenching; ~constituent quark scaling. 3) But: In the absence of a direct signal of deconfinement revealed by experiment alone, a QGP discovery claim must rest on the comparison with a theoretical framework. In this circumstance, further work to establish clear predictive power and provide In order to rely on theoryof fortheoretical compellinguncertainties QGP discoveryisclaim, we quantitative assessments necessary need: greaterappealing coherence; fewer adjusted parameters; quantitative for the present picture to survive as a lasting one. estimates of theoretical uncertainties 16

T. Hallman SC MTG Jan 200 Backup Slides 17

T. Hallman SC MTG Jan 200 Critical Future Exp’t Needs: Short-Term (some data already in the bag from run 4) Establish v 2 scaling more definitively: better statistics, more particles (incl. , , resonances), include correlations in recomb. -model fits. Establish that jet quenching is an indicator of parton, not hadron, E loss: higher p. T; better statistics dihadron correlations vs. reaction plane; away-side punchthrough? charmed meson suppression? Extend RHIC Au+Au meas’ments down toward SPS energy, search for possible indicators of a rapid transition in measured properties: determine turn-on of jet suppression vs. s; pp reference data crucial. Measure charmonium yields + open charm yields and flow, to search for signatures of color screening and partonic collectivity: charmed hadrons i chem. equil. ? Coalescence vs. fragmentation? D-meson flow; J/ suppression? (eventually , other “onia”) Measure hadron correlations with far forward high-energy hadrons in d+Au: search for monojet signature of interaction with classical gluon field. 18

T. Hallman SC MTG Jan 200 Some Critical Future Exp’t Needs: Longer-Term Develop thermometers for the early stage of the collision, when thermal equilibrium is first established: direct photons ( HBT for low E), thermal dileptons. Quantify parton E loss by measurement of mid-rapidity jet fragments tagged by hard direct photon, a heavy-quark hadron, or a far forward energetic hadron: constrain E loss of light quarks vs. heavy quarks vs. gluons in bulk matter. Test quantitative predictions for elliptic flow in U+U collisions: Considerable extrapolation away from Au+Au significant test for hydro predictive power @ RHIC. Measure hadron multiplicities, yields, correlations and flow at LHC & GSI, and compare to quantitative predictions based on models adjusted to work at RHIC: test viability and falsifiability of QGP-based theoretical framework. Devise tests for the fate of fundamental QCD symmetries in RHIC collision matter: chiral & UA(1) restoration? CP violation? Look especially at the strongly affected particles opposite a high-p. T 19

Soft-Hard Correlations: Partial T. Hallman Approach Toward Thermalization? SC MTG Jan 200 Leading hadrons Medium STAR PRELIMINARY { Closed symbols 4 < p. Ttrig < 6 Ge. V/c Open symbols 6 < p. Ttrig < 10 Ge. V/c { s. NN = 200 Ge. V Au+Au results: Assoc. particles: 0. 15 < p. T < 4 Ge. V/c Away side not jet-like! In central Au+Au, the balancing hadrons are greater in number, softer in p. T, and distributed ~statistically [~ cos( )] in angle, relative to pp or peripheral Au+Au. away-side products seem to approach equilibration with bulk medium traversed, making thermalization of the bulk itself quite plausible. 20

T. Hallman SC MTG Jan 200 Five Pieces of Important Evidence Early thermalization Statistical model Ideal hydro u, d, s equilibration near Tcrit CGC Very high anticipated initial gluon density + soft EOS Quark recombination constituent q d. o. f. p. QCD parton E loss Very high inferred initial gluon density 21