Evidence for the Production of the Quark-Gluon Plasma at RHIC

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

2. 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: • non-interacting quarks and gluons • 1st- or 2nd-order phase transition • evidence of chiral symmetry restoration A Definition of the Quark-Gluon Plasma This definition is consistent within the community and over time

3. z y x Anisotropic Flow py px Elliptic Flow at RHIC Anisotropic (Elliptic) Transverse Flow • The overlap region in peripheral collisions is not symmetric in coordinate space • 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 initial spatial anisotropy into the observed momentum anisotropy • Perform a Fourier decomposition of the momentum space particle distributions in the x-y plane • v2 is the 2nd harmonic Fourier coefficient of the distribution of particles with respect to the reaction plane Peripheral Collisions

4. Soft Sector: Evidence for Thermalization and EOS with Soft Point? Hydro calculations: Kolb, Heinz and Huovinen • Systematic m-dependence of v2(pT) suggests common transverse vel. field • mT spectra and v2 systematics for mid-central collisions at low pT 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 v2 and spectra for therm < 1 fm/c and soft (mixed-phase- dominated) EOS ~ consistent with LQCD expectations for QGP  hadron

5. How Unique & Robust is Hydro Account in Detail? P. Kolb, J. Sollfrank, and U. Heinz, Phys. Rev. C. C62 054909 (2000). • Are we sure that observed v2 doesn’t result alternatively from harder EOS (no transition) and late thermalization? • 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? • What has to be changed to understand HBT (below), and what effect will that change have on soft EOS conclusion? Sharp freezeout  dip Hydro+RQMD  no dip? Hydro vs. STAR HBT Rout/Rside Teaney, Lauret & Shuryak

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

7. Hard Sector: Evidence for Parton Energy Loss in High Density Matter PHENIX • Inclusive hadron and away-side cor-relation 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.

8. Questions for Parton Energy Loss Models • pQCD parton energy loss fits to observed central suppression  dNgluon/dy ~ 1000 at start of rapid expansion, i.e., ~50 times cold nuclear matter gluon density. • ~pT-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 pQCD models account for orientation- dependence of di-hadron correlation? Should be sensitive to both path length and matter expansion rate variation with (R).

9. Soft Sector: Hadron Yield Ratios Strangeness Enhancement Resonances STAR PHENIX • pT-integrated yield ratios in central Au+Au collisions consistent with Grand Canonical stat. distribution @ Tch = (160 ± 10) MeV, B  25 MeV, across u, d and s sectors. • Inferred Tch consistent with Tcrit (LQCD)  T0 >Tcrit . • Does result point to thermodynamic and chemical equilibration, and not just phase-space dominance?

10. Intermediate pT: Hints of Relevant Degrees of Freedom • For 1.5 < pT <6 GeV/c, see clear meson vs. baryon (rather than mass-dependent) differences in central-to-mid-central yields and v2. • v2/nq vs. pT /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

11. Questions for Coalescence Models Duke-model recomb. calcs. Duke-model recomb. calcs. • Can one account simultaneously for spectra, v2 and di-hadron  correlations at intermediate pT 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 fragment-ation treatments compatible? Double-counting, mixing d.o.f., etc.? • Do coalescence models have predictive power? E.g., can they predict centrality-dependences?

12. Gluon Saturation: a QCD Scale for Initial Gluon Density + Early Thermaliz’n Mechanism? Saturation model curves use optical Glauber sNN = 130 GeV 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 (con- strained by HERA e-p), Color Glass Condensate approaches ~account for RHIC bulk rapidity densities  dNg/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 dNch/d(sNN) so smooth?

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

14. But What We Observe (at least in the soft sector) Appears Smooth : HBT parameters Charged particle pseudo-rapidity density pT-integrated elliptic flow, scaled by initial spatial eccentricity pT-integrated elliptic flow 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.

15. The Five Pillars of RHIC Wisdom Ideal hydro Early thermalization + soft EOS Statistical model Quark recombination  constituent q d.o.f. …suggest appealing QGP-based picture of RHIC collision evolu-tion, BUT invoke 5 distinct models, each with own ambigu-ities, to get there. u, d, s equil-ibration near Tcrit pQCD parton E loss CGC Very high inferred initial gluon density Very high anticipated initial gluon density

16. Summary on QGP Search • All indications are that a qualitatively new form of matter is being • produced in central AuAu collisions at RHIC • The extended reach in energy density at RHIC appears to reach simplifying conditions in central collisions --~ideal fluid expansion; approx. local thermal equilibrium. • The Extended reach in pT at RHIC gives probes for behavior inaccessible at lower energies– jet quenching; ~constituent quark scaling. • 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 quantitative assessments of theoretical uncertainties is necessary for the present appealing picture to survive as a lasting one. In order to rely on theory for compelling QGP discovery claim, we need:greater coherence; fewer adjusted parameters; quantitative estimates of theoretical uncertainties

17. Backup Slides

18. Critical Future Exp’t Needs: Short-Term (some data already in the bag from run 4) Establish v2 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 pT; 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 in chem. equil.? Coalescence vs. frag-mentation? D-meson flow; J/ sup-pression? (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.

19. 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-pT hadron tag.

20. Soft-Hard Correlations: Partial Approach Toward Thermalization? Leading hadrons { s = 200 GeV Au+Au results: Assoc. particles: 0.15 < pT < 4 GeV/c Closed symbols  4 < pTtrig < 6 GeV/c Open symbols  6 < pTtrig < 10 GeV/c { NN Medium STAR PRELIMINARY Away side not jet-like! In central Au+Au, the balancing hadrons are greater in number, softer in pT, 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.

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