Quark Gluon Plasma in Relativistic Heavy Ion Collisions

1. Quark Gluon Plasma in Relativistic Heavy Ion Collisions Prospects on Fundamental Physics in the 21st Century Barbara V. Jacak Stony Brook Feb.18, 2004

2. outline • Creating hot, dense matter in the laboratory • at the Relativistic Heavy Ion Collider • What RHIC data are telling us: • Pressure, thermalization • Energy transport in the medium • “control experiment” d+Au • Hadronization properties • Implications for Quark Gluon Plasma • Conclusions and prospects

3. The Physics of Heavy Ion Collisions • Create very high temperature and density matter • as existed ~1 msec after the Big Bang • inter-hadron distances comparable to that in neutron stars • collide heavy ions to achieve maximum volume • Study the hot, dense medium • is thermal equilibrium reached? • transport properties? equation of state? • do the hadrons dissolve into a quark gluon plasma? • fundamental properties of matter: goal of the QUESTS center! • Collide Au + Au ions at high energy • s = 200 GeV/nucleon pair, p+p and d+A to compare • Also polarized p+p collisions to study carriers of p’s spin

4. + +… Study Quantum Chromo Dynamics at RHIC • Color charge of gluons  they interact among themselves • theory is non-abelian • curious properties at long distance, including confinement short distance: force is weak (probe w/ high Q2, calculate with perturbation theory) large distance: force is strong (probe w/ low Q2, calculations must be non-perturbative) High temperature: force becomes screened by produced color-charges (gets weak)

5. Must create probes in the collision itself: predictable quantity, interact differently in QGP vs. hadron matter fast quarks/gluons, J/Y, D mesons thermal radiation vacuum QGP did something new happen at RHIC? • Study collision dynamics (via final state) • Probe the early (hot) phase Equilibrium? hadron spectra, yields Collective behavior? i.e. pressure and expansion elliptic, radial flow

6. p-p PRL 91 (2003) 241803 Good agreement with NLO pQCD Parton distribution functions Fragmentation functions Study simple complex systems: p+p, “p”+A, A+A collisions start with pQCD & pp collisions: itworks! Have a handle on initial NN interactions by scattering of q, g inside N We also need: p0

7. EOS Tc ~ 170 ± 10 MeV (1012 °K) e ~ 3 GeV/fm3 Lattice QCD says we must create these conditions to study quark gluon plasma QGP physics is non-perturbative: calculate on lattice we look for physics beyond simple superposition of NN at low momentum/large distance scales: Equilibration Collective effects Energy, color transport in dense medium Deconfinement? Karsch, Laermann, Peikert ‘99 e/T4 T/Tc

8. history of a heavy ion collision e, pressure builds up g, g* e+e-, m+m- p, K, p, n, f, L, D, X, W, d, Hadrons reflect thermal properties when inelastic collisions stop (chemical freeze-out). Real and virtual photons from quark scattering most sensitive to the early stages. Probe also with q, g scattered (hard) out of incoming nucleons we focus on mid-rapidity (y=0), as it is the CM of colliding system 90° in the lab at collider

9. RHIC at Brookhaven National Laboratory RHIC is first dedicated heavy ion collider 10 times the energy previously available!

10. STAR 4 complementary experiments

11. Energy  to beam direction per unit velocity || to beam pR2 2ct0 Is the energy density high enough? PRL87, 052301 (2001) Colliding system expands: • e 5.5 GeV/fm3 (200 GeV Au+Au) YES - well above predicted transition!

12. ( pQCD x Ncoll) / background Vogelsang/CTEQ6 ( pQCD x Ncoll) / (background x Ncoll) [w/ the real suppression] [if there were no suppression] pQCD in Au+Au? direct photons At high pT, it also works! gTOT/gp0 Au+Au 200 GeV/A: 10% most central collisions Preliminary pT (GeV/c) []measured / []background = measured/background

13. Almond shape overlap region in coordinate space pressure: a barometer called “elliptic flow” Origin: spatial anisotropy of the system when created, followed by multiple scattering of particles in the evolving system spatial anisotropy  momentum anisotropy v2: 2nd harmonic Fourier coefficient in azimuthal distribution of particles with respect to the reaction plane

14. Preliminary STAR STAR Preliminary v2 measured by the experiments 200 GeV: 0.2< pt < 2.0 130 GeV: 0.075< pt < 2.0 200 GeV: 0.150< pt < 2.0 4-part cumulants v2=0.05 200 GeV: Preliminary - Consistent results - At 200 GeV better pronounced decrease of v2 for the most peripheral collisions.

15. Kolb, et al Hydro can reproduce magnitude of elliptic flow for p, p. BUT must add QGP to hadronic EOS!! Similar conclusion reached by Ko, Kapusta, Bleicher, others… v2 predicted by hydrodynamics • see large pressure buildup • anisotropy  happens fast • v2 reproduced by hydro  • early equilibration ! Hydro. Calculations Huovinen, P. Kolb, U. Heinz STAR PRL 86 (2001) 402 central

16. How to get fast equilibration & large v2? Huge cross sections!!

17. Evidence for equilibrated final state Calculate hadron yields in Grand Canonical ensemble Observed hadron ratios in agreement with thermal ratios! T(chemical freeze-out) ~ 175 MeV

18. early universe 250 RHIC 200 quark-gluon plasma 150 SPS Lattice QCD AGS deconfinement chiral restauration thermal freeze-out 100 SIS hadron gas 50 neutron stars atomic nuclei 0 0 200 400 600 800 1000 1200 Baryonic Potential B [MeV] Locate RHIC on phase diagram From fit of yields vs. mass (grand canonical ensemble): Tch = 176 MeV mB = 41 MeV These are the conditions when hadrons stop interacting T Observed particles “freeze out” at/near the deconfinement boundary!

19. schematic view of jet production hadrons leading particle q q hadrons leading particle Jets: calibrated probe of the medium Jets of hadrons arise from hard scattered quarks Observed via fast leading particles or azimuthal correlations between the leading particles But, before they create jets, the scattered quarks radiate energy (~ GeV/fm) in the colored medium  decreases their momentum  fewer high momentum particles  beam  “jet quenching”

20. nucleons Technique to search for jet quenching • Compare to baseline: nucleon-nucleon collisions at the • same energy • To 0’th order: Au + Au collisions start with collisions of quarks & gluons in the individual N-N reactions • (+ effects of • nuclear binding and • collective excitations) • Hard scattering (high momentum transfer) processes scale as the number of N-N binary collisions <Nbinary> • so for high momentum expect: YieldA-A = YieldN-N. <Nbinary>

21. AA AA If no “effects”: RAA < 1 in regime of soft physics RAA = 1 at high-pT where hard scattering dominates Suppression: RAA < 1 at high-pT AA Nuclear Modification of Hadron Spectra? 1. Compare Au+Au to nucleon-nucleon cross sections 2. Compare Au+Au central/peripheral Nuclear Modification Factor: nucleon-nucleon cross section <Nbinary>/sinelp+p

22. Au-Au s = 200 GeV: high pT suppressed! PRL91, 072301(2003)

23. near side away side Back-to-back jets are suppressed in central collisions! peripheral central can look for the jet on the other side STAR PRL 90, 082302 (2003) Peripheral Au + Au Central Au + Au

24. STAR Preliminary near-side partner away-side partner Away side jet partners shifted to lower pT Exactly what is expected if the jet loses energy & multiple scatters in the medium Multiple scatteringthermalization Partonic or hadronic? unclear Where does the lost energy go? preliminary Away side jet broadens in more central Au+Au

25. See jet suppression! a final state effect? Hadron gas • Hadronic absorption of fragments: • Gallmeister, et al. PRC67,044905(2003) • Fragments formed inside hadronic medium • Energy loss of partons in dense matter • Gyulassy, Wang, Vitev, Baier, Wiedemann… Absent in d+Au collisions! d+Au is the “control” experiment

26. probe rest frame r/ ggg Suppression: an initial state effect? • Gluon Saturation • (color glass condensate) Wavefunction of low x gluons overlap; the self-coupling gluons fuse, saturating the density of gluons in the initial state.(gets Nch right!) • Multiple elastic scatterings (Cronin effect) Wang, Kopeliovich, Levai, Accardi • Nuclear shadowing Levin, Ryshkin, Mueller, Qiu, Kharzeev, McLerran, Venugopalan, Balitsky, Kovchegov, Kovner, Iancu … RdAu~ 0.5 D.Kharzeev et al., hep-ph/0210033 Broaden pT :

27. Experiments show NO suppression in d+Au! PHENIX Preliminary p0 STAR Preliminary PHOBOS Preliminary

28. Centrality Dependence Au + Au Experiment d + Au Control PHENIX preliminary • Dramatically different and opposite centrality evolution of AuAu experiment from dAu control. • Jet Suppression is clearly a final state effect.

29. Medium effects on hadronization? • Hadronization of intermediate momentum partons (3-6 GeV/c) takes place in/near the hot, quark-rich medium • Might expect hadronization to involve quarks from the medium, in addition to those from vacuum • Experimentally study via • Relative abundance of different hadrons at high pT • Correlations of jet fragments • Flavor composition of jet fragments

30. R. Fries, et al pQCD spectrum shifted by 2.2 GeV Hydro. expansion at low pT + jet quenching at high pT: Recombination of boosted quarks? yields many high pT baryons Teff = 350 MeV Particle mix in central AuAu is different! PRL 91 (2003) 172301 p/ ~1 at high pT in central collisions Higher than in p+p or jets in e+e- collisions

31. Are baryons from jets suppressed? Baryons look not suppresed  Ncoll at pT = 2 – 4 GeV/c Mesons, including f are suppressed  Effect governed by quark content

32. So, are the baryons soft, or from jets? • Look for jet-like correlations with baryons of pT = 2.5 - 4 GeV/c Identify trigger particle Count associated particles per trigger • If baryon excess from quark recombination (coalescence) Expect fewer jet-like associated particles thermal partons coalescence  no partner So yield of associated particles should decrease when coalescence contribution increases with centrality.

33. jet partner equally likely for trigger baryons & mesons • slight decrease of baryon associated particles with centrality! • expected from recombination The data say: QM04 consensus: need coalescence of jet + thermal partons this is medium modification of the jet fragmentation!

34. What about heavy quarks? • J/Y • Test confinement: do bound c + c survive? • or does QGP screening kill them? • Open Charm • Extra heavy quarks from hot, dense gluon gas? • Do the c quarks lose energy like the light quarks? Need (a lot) more statistics (currently being collected) But can take a first look…

35. J/Y suppression was observed at CERN at s=18 GeV/A NA50 collaboration J/Y yield Fewer J/Y in Pb+Pb than expected! Interpret as color screening of c-cbar by the medium Initial state processes affect J/Y too so interpretation heavily debated...

36. PHENIX looks for J/Y  e+e- and m+m- need electron/ pion separation at the level of one in 10,000 (needle in a haystack!) There is the electron. Ring Imaging Cherenkov counter to tag electrons “RICH” See signal when vpart. > cmedium

37. 0-20% most central Ncoll=779 40-90% most central Ncoll=45 20-40% most central Ncoll=296 R.L. Thews, M. Schroedter, J. Rafelski Phys. Rev. C63 054905 (2001): Plasma coalesence model for T=400MeV and ycharm=1.0,2.0, 3.0 and 4.0. L. Grandchamp, R. RappNucl. Phys. A&09, 415 (2002) and Phys. Lett. B 523, 50 (2001): Nuclear Absorption+ absoption in a high temperature quark gluon plasma J/Y at RHIC Look at J/Y nucl-ex/0305030 A. Andronic et. Al. Nucl-th/0303036 Proton Don’t know yet about deconfinement, but don’t see EXTRA (thermal) J/Y

38. Open charm: baseline is p+p collisions PHENIX PRELIMINARY Measure charm s via semi-leptonic decay to e+ & e- p0, h, photon conversions are measured and subtracted fit p+p data to get the baseline for d+Au and Au+Au.

39. No large suppression as for light quarks! PHENIX PRELIMINARY Curves are the p+p fit, scaled by the number of binary collisions

40. Implications of the results for QGP • Ample evidence for equilibration • Hydrodynamics reproduces elliptic flow, hadron spectra • Yields  chemical equilibrium • Success of quark coalescence  hadronization of thermalized, expanding q,qbar medium • v2 & jet quenching measurements constrain initial gluon density, energy density, and energy loss

41. Medium properties • Extract by constraining QCD-inspired models with measured jet suppression and v2 • Find (values from Vitev, et al; others consistent)

42. Implications of the results for QGP • Ample evidence for equilibration • initial dN(gluon)/dy ~ 1000, energy density ~ 15 GeV/fm3, energy loss ~ 7-10 GeV/fm • Very rapid, large pressure build up requires • parton interaction cross sections 50x perturbative s

43. How to get 50 times pQCD s? spectral function • Lattice indicates that hadrons don’t all melt at Tc! • hc bound at 1.5 Tc Asakawa & Hatsuda, PRL92, 012001 (2004) • charmonium bound states up to ~ 1.7 Tc Karsch; Asakawa&Hatsuda • p, s survive as resonances Schaefer & Shuryak, PLB 356 , 147(1995) • q,g have thermal masses at high T. as runs up at T>Tc? (Shuryak and Zahed) • would cause strong rescattering qq  meson

44. E. Shuryak

45. Implications of the results for QGP • Ample evidence for equilibration • v2 & jet quenching measurements constrain initial gluon density, energy density, and energy loss • parton interaction cross sections 50x perturbative s • parton correlations at T>Tc • complicates cc bound states as deconfinement probes! • Hadronization by coalescence of thermal,flowing quarks • v2 & baryon abundances point to quark recombination as hadronization mechanism • Jet data imply must also include recombination between quarks fromjets and the thermalized medium •  medium modifies jet fragmentation!

46. No CGC signal at mid-rapidity So, perhaps Rda G-sat. Rda pQCD >2 BFKL, DGLAP Xc(A) RHIC Pt (GeV/c) Pt (GeV/c) Log Q2 How about Color Glass Condensate? Central: Enhanced not suppressed PHENIX preliminary y=0 Peripheral d+Au (like p+p)

47. d Au PhenixPreliminary But at forward rapidity reach smaller x y = 3.2 in deuteron direction  x  10-3 in Au nucleus Strong shadowing, maybe even saturation?

48. Statistically it’s a 4 effect Systematic Error under study Efficiency being evaluated Nobody scrambles quarks like we do! Anti-Penta Quarks with PHENIX? Q- n + K- 1.54 GeV

49. conclusions • Rapid equilibration at RHIC; hydrodynamics works • EOS is not hadronic but do not have perturbative plasma! • Parton scattering cross section is very large • The hot matter is “sticky” – absorbs & transports energy • Modifies fragmentation function of moderate energy jets • Color Glass Condensate? Maybe at x ~ 10-3 (y>2) • QGP discovery? • My personal answer: yes! But - what’s in a name? • Need at RHIC: Tinitial (thermal g, dileptons) & J/Y Need from theory: consistent model of many observables • extract table of parameters such as from WMAP! • Both are on the way…

50. v2 recombination of flowing quarks nucl-ex/0305013 • p above p for • pT < 2 GeV/c, in agreement with hydrodynamics • Then crosses over. • Values ~ saturate • at high pT • geometry? • v2/quark ~ constant •  create hadrons • by coalescence of • quarks from • boosted distribution