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

2. T. Hallman SC MTG Jan 2005 2 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:  non-interacting quarks and gluons  1 st - or 2 nd -order phase transition  evidence of chiral symmetry restoration This definition is consistent within the community and over time

3. T. Hallman SC MTG Jan 2005 3 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 v 2 is the 2 nd harmonic Fourier coefficient of the distribution of particles with respect to the reaction plane Anisotropic Flow x y z pxpx pypy Anisotropic (Elliptic) Transverse Flow Elliptic Flow at RHIC Peripheral Collisions

4. T. Hallman SC MTG Jan 2005 4 Soft Sector: Evidence for Thermalization and EOS with Soft Point?  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-phase- dominated) EOS ~ consistent with LQCD expectations for QGP  hadron Hydro calculations: Kolb, Heinz and Huovinen

5. T. Hallman SC MTG Jan 2005 5 How Unique & Robust is Hydro Account in Detail? P. Kolb, J. Sollfrank, and U. Heinz, Phys. Rev. C. C62 054909 (2000). Sharp freezeout  dip Hydro+RQMD  no dip? Teaney, Lauret & Shuryak Hydro vs. STAR HBT R out /R side  Are we sure that observed v 2 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?

6. T. Hallman SC MTG Jan 2005 6 /  inel p+p nucleon-nucleon cross section Nuclear Modification Factor: AA hadrons leading particle suppressed q q ? If R = 1 here, nothing new going on Self-Analyzing (High p T ) Probes of the Matter at RHIC

7. T. Hallman SC MTG Jan 2005 7 Hard Sector: Evidence for Parton Energy Loss in High Density Matter  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. PHENIX

8. T. Hallman SC MTG Jan 2005 8 Questions for Parton Energy Loss Models  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 ).  pQCD parton energy loss fits to observed central suppression  dN gluon /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?

9. T. Hallman SC MTG Jan 2005 9 Soft Sector: Hadron Yield Ratios Strangeness Enhancement Resonances STAR PHENIX  p T -integrated yield ratios in central Au+Au collisions consistent with Grand Canonical stat. distribution @ T ch = (160 ± 10) MeV,  B  25 MeV, across u, d and s sectors.  Inferred T ch consistent with T crit (LQCD)  T 0 >T crit.  Does result point to thermodynamic and chemical equilibration, and not just phase-space dominance?

10. T. Hallman SC MTG Jan 2005 10 Intermediate p T : Hints of Relevant Degrees of Freedom  For 1.5 < p T <6 GeV/c, see clear meson vs. baryon (rather than mass-dependent) differences in central-to-mid- central yields and v 2.  v 2 /n q vs. p T /n q 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. T. Hallman SC MTG Jan 2005 11 Questions for Coalescence Models  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 fragment- ation treatments compatible? Double-counting, mixing d.o.f., etc.?  Do coalescence models have predictive power? E.g., can they predict centrality-dependences? Duke-model recomb. calcs.

12. T. Hallman SC MTG Jan 2005 12 Gluon Saturation: a QCD Scale for Initial Gluon Density + Early Thermaliz’n Mechanism?  s NN = 130 GeV Au+Au Saturation model curves use optical Glauber  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  dN g /dy ~ consistent with parton E loss.  How robust is agreement, given optical vs. MC Glauber ambiguity in calcu -lating N part, and assumption of ~one charged hadron per gluon?  CGC applies @ SPS too? If not, why is measured dN ch /d  (  s NN ) so smooth?

13. T. Hallman SC MTG Jan 2005 13 Lattice QCD Predicts Some Sort of RAPID Transition! in entropy density, hence pressure in heavy-quark screening mass in chiral condensate 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 T c  160 – 170 MeV.

14. T. Hallman SC MTG Jan 2005 14 But What We Observe (at least in the soft sector) Appears Smooth : 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. Charged particle pseudo- rapidity density HBT parameters p T -integrated elliptic flow p T -integrated elliptic flow, scaled by initial spatial eccentricity

15. T. Hallman SC MTG Jan 2005 15 …suggest appealing QGP-based picture of RHIC collision evolu- tion, BUT invoke 5 distinct models, each with own ambigu- ities, to get there. pQCD parton E loss The Five Pillars of RHIC Wisdom Ideal hydro Quark recombination  constituent q d.o.f. CGC Statistical model Early thermalization + soft EOS Very high inferred initial gluon density Very high anticipated initial gluon density u, d, s equil- ibration near T crit

16. T. Hallman SC MTG Jan 2005 16 Summary on QGP Search All indications are that a qualitatively new form of matter is being produced in central AuAu 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. 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. T. Hallman SC MTG Jan 2005 17 Backup Slides

18. T. Hallman SC MTG Jan 2005 18 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 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. T. Hallman SC MTG Jan 2005 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 & U A (1) restoration? CP violation? Look especially at the strongly affected particles opposite a high-p T hadron tag.

20. T. Hallman SC MTG Jan 2005 20 Soft-Hard Correlations: Partial Approach Toward Thermalization? Leading hadrons Medium STAR PRELIMINARY  s = 200 GeV Au+Au results: NN Closed symbols  4 < p T trig < 6 GeV/c Open symbols  6 < p T trig < 10 GeV/c { { Assoc. particles: 0.15 < p T < 4 GeV/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.

21. T. Hallman SC MTG Jan 2005 21 pQCD parton E loss Five Pieces of Important Evidence Ideal hydro Quark recombination  constituent q d.o.f. CGC Statistical model Early thermalization + soft EOS Very high inferred initial gluon density Very high anticipated initial gluon density u, d, s equilibration near T crit