Searching for Quark-Gluon Plasma in Relativistic Nucleus-Nucleus Collisions

1. Searching for Quark-Gluon Plasma in Relativistic Nucleus-Nucleus Collisions Tim Hallman ICPAQGP5 Kolkata, India February 8-12, 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

3. z y x Anisotropic Flow py px The Strongest Evidence For (Locally)Thermalized State of Matter and EOS with a soft point : Observed Elliptic Flow vs the Predictions of Hydro Peripheral Collisions Hydro calculations: Kolb, Heinz and Huovinen

4. Soft Sector: Evidence for Thermalization and EOS with Soft Point? What do the v2 and Hydro results tell us ? • 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 and high initial energy density (e > 10 GeV/fm3) • Best agreement with v2 and spectra for therm < 1 fm/c and soft (mixed-phase- dominated) EOS ~ consistent with LQCD expectations for QGP  hadron What do we need to understand better ? • Real sensitivity of the Hydro predictions to the EOS and the Freeze-out Treatment

5. Particle multiplicity ratios P. Braun-Munzinger, D. Magestro, J. Stachel & K.R Supporting Evidence: Hagedorn Resonance Gas and Particle Multiplicity Ratios at RHIC MODEL DATA • 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.

6. Resonance gas model provides good description of particle yields and their ratios in heavy ion collisions from AGS up to RHIC Together with the data at lower energy… 40 GeV/u Pb+Pb T = 148 MeV B = 400 MeV PBM,Stachel, Redlich Nucl-th 0304013 For recent review of RHIC, e.g., M. Kaneta & N. Xu, W. Broniowski & W. Florkowski, O. Baranikowa et al. For recent review of SPS e.g., F. Becattini, M. Gazdzicki, A. Keranen, J. Manninen & R. Stock; P. Braun-Munzinger, J. Stachel & K.Redlich

7. What do the results tell us? Chemical freeze-out at RHIC and top SPS energy coincides with phase boundary predicted by LQCD Data are nearly described by curve of constant critical energy density Open Questions: Where is the phase boundary at lower energy? Could it be that the full chemical freeze-out curve is coincident with the phase boundary?

8. Partonic radiative energy loss in dense matter as a means to (indirectly) test deconfinement Thick plasma (Baier et al.): Gluon bremsstrahlung Thin plasma (Gyulassy et al.): • Linear dependence on gluon density glue: • measure DE  gluon density at early hot, dense phase • High gluon density requires deconfined matter • (“indirect” QGP signature !)

9. leading particle suppressed hadrons q q ? Experimental Tools: High pT (Self-Analyzing) 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

10. A comment on the sensitivity of RAA K.J. Eskola, H. Honkanken, C.A. Salgado, U.A. Wiedemann, hep-ph/0406319 • Quenching so strong that RAA loses sensitivity to the density of the medium: dominated by unquenched “halo” • Increased sensitivity only through detailed angular correlations and/or decreasing coupling strength Dainese, C. Loizides, G. Paic, hep-ph/0406201

11. D. d’Enterria High pT yields in central Au+Au are suppressed Binary Collision scaling x5 Factor 5 suppression: huge effect!

12. 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.

13. Parton energy loss ? RAA data vs GLV model • Au+Au suppression (I. Vitev and M. Gyulassy, hep-ph/0208108) and d+Au enhancement (I. Vitev, nucl-th/0302002 understood in an approach that combines multiple scattering with absorption in a dense partonic medium • Medium induced radiative energy loss is the only currently known physical mechanism that can consistently explain the high pT suppression. • From GLV model, initial gluon density dng/dy~1000 is obtained. This corresponds to an initial energy density e~15 GeV/fm3. • These values are consistent with the energy density obtained from the dET/dh measurement as well as ones from the hydro models.

14. What do the data tell us? • 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. What do we need to understand better? How sensitive is this quantitative conclusion to assumptions concerning: • factorization in-medium • vacuum fragmentation following degradation; • treatments of expansion • initial-state cold energy loss • Can pQCD models account for orientation- dependence of di-hadron correlation?

15. A related question: the initial condition • Large nucleus (A) at low momentum fraction x  gluon distribution saturates ~ 1/as(QS2) with QS2~ A1/3 • A collision puts these gluons ‘on-shell’ r ~ A xg(x,Q2) / R2 • Parton-hadron duality maps gluons directly to charged hadrons • Parton dynamics in a dense system of gluons differs from pQCD • Saturated gluon density ( CGC )  effective field theory of dense gluon systems provides an appropriate description of the initial condition D. Kharzeev, E. Levin and L. McLerran, Phys. Lett. B 561(2003) 93

16. dN/dh / .5Npart Npart Predicted Consequences: Saturation in Multiplicity STAR sNN = 130 GeV Au+Au sNN = 130 GeV Au+Au • Suppression of RdAu at forward h • Disagreement with expectations based on incoherent multiple scattering in the initial state (i.e. standard factorized pQCD explanation for Cronin enhancement)  suppression of high pT • in d+Au at small x

17. Comparison of CGC calculations to data K. Tuchin Open Charm Charmonium Inclusive h- Inclusive hadrons

18. Comparison of CGC Calculations to the Data C. Salgado: AA Should the model work at SPS energy ?

19. Gluon Saturation: What do the data indicate? • 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  dNg/dy ~ consistent with parton E loss. What do we need to understand better? How robust is agreement, and how sensitive to assumption of ~one charged hadron per gluon?

20. Can the boundaries on this diagram be mapped out experimentally? p0 “Mono-jet” PT is balanced by many gluons Dilute parton system (deuteron) p0 Beam View Top View • Ep > 25 GeV •   4 Dense gluon field (Au) Large Dhp0+h± correlations f • Suppressed at small <xF> , <pT,p> • Consistent with CGC picture • Consistent in d+Au and p+p at larger <xF> and <pT,p> • as expected by HIJING 25<Ep<35GeV STAR Preliminary Fixed h, as E & pT grows Fixed h, as E & pT grows 35<Ep<45GeV Statistical errors only Strengthening the proof with particle correlations DIS’04

21. Something that was a surprise.. Anomalous p/p ratio in 2-4 GeV/c Proton scales with Ncoll,Mesons don’t The first indication that a new mechanism other than universal parton fragmentation is the dominant source of baryons in the intermediate pT range. PHENIX PHENIX  = 2.2  = 0 BRAHMS

22. Also seen and extended to higher pT with topological PID in STAR PHENIX STAR preliminary Observations: • Simple fragmentation picture fails for pT less than ~6 GeV/c • p+pbar/h enhancement in Au + Au not fully explained by Cronin effect • Strong baryon/meson modification in Au + Au also in L/K0s ratio

23. Extending to the strange sector: RCP of Strange Hadrons • Two groups (2<pt<6GeV/c): - K0s, K, K*, f mesons - L, X, W baryons • dependence on number of valence quarks • limited to pt<6GeV/c ? • hadron production from quark recombination/ coalescence ?

24. Recombination Extended to Elliptic Flow The complicated observed flow pattern in v2(pT) for hadrons d2n/dpTdf ~ 1 + 2 v2(pT) cos (2 f) is predicted to be “simple’ at the quark level underpT → pT / n , v2 → v2 / n , n = (2, 3) for (meson, baryon) if the flow pattern is established at the quark level

25. A possible explanation: Coalescence + Fragmentation Duke-model recomb. calcs. Duke-model recomb. calcs.

26. Is recombination the complete answer? Jet correlation of paticles associated with a leading proton and meson • The “jet-like” correlations of particles with leading baryon rule out the simplest recombination models, which assume a perfectly thermal source of partons. • At present, no model provides a complete understanding of hadron formation in the intermediate pT regime. Associated yields similar in all cases PHENIX nucl-ex/0408007 |η|<0.7 |η|<0.35 Dominance of jet-like production Inconsistent (?) with different suppression for mesons/baryons

27. Intermediate pT: Hints of Relevant Degrees of Freedom ? What do the data indicate? • 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 suggests constituent-quark scaling. If better established experimentally, this would give direct evidence of degrees of freedom relevant at hadronization, and suggest collective flow at the constituent quark level (NB: constituent quarks ≠ partons). What do we need to understand better? • 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 fragmentation treatments compatible? Double-counting, mixing d.o.f., etc.?

28. Where will some of the next insights come from?

29. Direct photons from PHENIX p+p Photons scale as binary collisions while p0 are suppressed: consistent with energy loss picture Good agreement between p+p baseline measurements and NLO pQCD

30. Direct photons as a thermometer WA98: Phys. Rev. Lett. 93, 022301(2004) • As yet, no measurement at RHIC directly sensitive to temperature at early times • Intriguing possibility: direct photons, especially with low pT interferometric methods • Potentially sensitive to thermal black-body radiation from the plasma

31. E866: PRL 84, 3256 (2000)NA3: ZP C20, 101 (1983) Klein,Vogt, PRL 91:142301,2003 Kopeliovich, NP A696:669,2001 Shadowing vs absorption in d+Au collisions hep-ph/0311048 • Data seem to show that shadowing • /absorption plays a (small) role when • comparing d-A to p-p • How much is shadowing? No x2 scaling • How much is nuclear absorption? •  Need more data

32. preliminary NA60: first results • Anxiously waiting for the centrality dependence…… • Should span the region across the onset of the anomalous suppression

33. First Look at Centrality Dependence of Continuum • Full systematic vs Npart • Dimuon spectra normalized to w ~ normalized to 1/Nch similar to CERES • Centrality dependence stronger than linear in Npart • In the IM region • Between r/w and f • Below r/w • Also visible - f enhancement • J/ suppression HP, Gianluca Usia

34. NA50 Pb-Pb • NA60 In-In158 GeV/nucleon pT > 1.1 GeV/c Statistical errors only First Look at Low Mass Pair region in In-In NA50 158 GeV/u In-In • Low mass continuum • High statistics from 2mmupward • Low pair acceptnce for low M low pT • Pair acceptance different from CERES • Clearly visible hadron decays • f → mm • w→ mm • h → mm BR: (5.8 +/- 0.8) 10-6) HP, Gianluca Usia • Accurately measured yields, slopes, and centrality dependence • No indication for medium modifications of f • mfindependent of Npart within few MeV

35. NA38/NA50 E.Phys.J. C13 (2000) 69 NA38/NA50 S-U Discovery of Low Mass Dilepton Enhancement CERES Phys.Lett.B422 (98) 405; Nucl.Phys.A661(99) 23 HELIOS-3 HELIOS-3 E.Phys.J. C13 (2000) 433 Di-lepton excess at low and intermediate masses well established

36. DLS puzzle Strong enhancement over hadronic cocktail with “free”  spectral function Enhancement not described by in-medium  spectral function Verification expected to come soon from HADES Connection of enhancement to SPS results not clear HADES C-C 1 AGEV free spectral function inmedium spectral fct. preliminary Dileptons Measured at Low Energies DLS Ca-Ca 1 AGeV

37. Heavy quark energy loss: U. Wiedemann, M. Djordjevic, X.-N.Wang

38. Kinetic Thermalization: Open Charm Flow Test mechanism for thermalization: charm heavy, so needs many collisions to reach kinetic equilibrium. Current measurements: indirect from electrons, and so suffer from large statistical and systematic errors; centrality dependence also needed. Need direct open charm reconstruction (at low pT)!

39. 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 Soft-Hard Correlations: Partial Approach Toward Thermalization? 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.

40. What have we learned (so far)? + We know that the matter is extremely dense and it thermalizesvery rapidly. First order estimates of the energy density from dET/d (a la Bjorken), Hydro, and jet suppression results are consistent and all well in excess of the density needed for a QGP predicted by LQCD (~ 10-15 GeV/fm3). But • There is (so far) no direct(unequivocal)evidence that • the matter is deconfined • the primary degree of freedom of the matter is that of quarks and gluons • the matter is at high temperature (T > 170 MeV) • We need a better understanding of the real sensitivity of Hydro to the EOS, and to improve its consistency in describing spectra, v2, and HBT. At present we can not draw quantitative conclusions on the properties of the matter such as the equation of state and the presence of a mixed phase.

41. What have we learned (so far)? The data appear to demand an explanation beyond a purely hadronic scenario: • The lower limit of the energy densities derived fromdET/dhare ~ 4-5 GeV/fm3: The hydro-models require early thermalization(ttherm< 1fm/c)and high initial energy densitye > 10 GeV/fm3 . Their success implies the matter is well described as ideal relativistic fluid • Initial gluon density dng/dy~1000 and initial energy densitye~15 GeV/fm3 are obtained from GLV model of jet quenching. A similarly high initial energy density is obtained by other models. All these estimates of energy density are well in excess of ~1 GeV/fm3 obtained in lattice QCD as the energy density needed to form a deconfined phase.

42. Conclusion • A qualitatively new form of matter is produced in central • relativistic nucleus-nucleus collisions! • But:Further work is needed to prove this is the quark-gluon plasma according to • our definition. • likely sources of insight on experimental side (in the near term): • soft sector: • open charm elliptic flow • v2 systematics (more particles, better statistics) • low mass di-leptons • low pT direct photons • jets and hard probes: • higher pt; search for away side punch-through • better statistics di-hadron correlations wrt reaction plane, • heavy quark suppression (energy loss) • search for forward mono-jets • study of suppression (?) for onium (screening in the plasma) • theory:provide quantitative assessments of sensitivity (e.g. to EOS) & theoretical • uncertainties; incorporate higher level effects (e.g. correlations into coalescence)