Recent developments in RHIC physics

1. Recent developments in RHIC physics Rudolph C. Hwa University of Oregon IHEP seminar June 14, 2005

2. Outline Major achievements in RHIC physics Outstanding puzzles Resolution of the puzzles Summary

3. Major achievements Hydrodynamical description of HIC Elliptic flow Jet quenching Strongly interacting Quark Gluon Plasma

4. Hydrodynamical description of HIC Condition for hydrodynamics to be valid: Locally thermalized -- fast Local conservation of energy & momentum Local conservation of charge current density Equation of motion Initial condition Final freeze-out Put in: to solve hydro eq

5. Results on single-particle distributions from hydro Kolb & Heinz, QGP3 equ=0.6 fm, Tinit=340 MeV, init=25 GeV/fm3

6. Df = 90° y Df = 0° x Requires early thermalization to build up pressure for expansion in the x direction Spatial eccentricity Momentum eccentricity Azimuthal anisotropy Non-central collision  dependence

7. Elliptic flow -- v2 Agree with data for pT<2GeV/c possible only if therm<1 fm/c Huovinen, Kolb, Heinz, Ruuskanen, Voloshin Phys. Lett. B 503 58, (2001).

8. larger flow Flattening of v2 flattening of spectrum for heavier particles Kolb & Rapp, PRC 2003 Heavier particles have more <pT>, and flatter slope. Good support for hydrodynamics for pT < 2 GeV/c.

9. sQGP Conclusion from hydrodynamics • Fireball thermalizes quickly (< 1 fm/c) • Initial energy density >> needed for quark deconfinement • Expansion governed by strong rescattering (ideal hydrodynamics) • QGP is formed, lives for 5-7 fm/c, and is strongly interactive (non-perturbative)

10. Hard scattering of partons -- pQCD +  Jets But hydrodynamics does not work for pT > 2 GeV/c pT < 2 GeV/c soft Traditional regions: pT > 2 GeV/c hard Soft region: macroscopic physics, collective variables. Hard region: microscopic physics, parton variables.

11. particle High pT Physics of Nuclear Collisions at High Energy Well studied for 20 years ---- pQCD What was a discovery yesterday is now used for calibration today. • In pp collisions discrepancy < 5% • In heavy-ion collisions there are problems involving factors of 10 to be understood.

14. To learn about the properties of a dense and hot medium of quarks (and gluons), we can only look at what come out: Au+Auhadrons, leptons, photons containquarks We need to know how to interpret the hadron spectra to learn about the quarks and gluons.

15. “Jets” via dihadron azimuthal distributions p+p  dijet • trigger: highest pT track • Df distribution: 2 GeV/c<pT<pTtrigger • normalize to number of triggers Phys Rev Lett 90, 082302 trigger

16. jet parton nucleon nucleon Jets in RHI Collisions Find this……….in this p+p jet+jet (STAR@RHIC) Au+Au ??? (STAR@RHIC)

17. Inclusive hadron suppression in Au+Au PHENIX pT (GeV) at different centralities PHOBOS

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

22. Is suppression an initial or final state effect? partonic energy loss Initial state? Final state? gluon saturation How to discriminate? Turn off final state d+Au collisions

23. d+Au yields are not suppressed STAR PHOBOS PRL 91, 072302/3/4/5 BRAHMS PHENIX Hadron suppression in central Au+Au is a final state effect

24. trigger Azimuthal dependence of jet production associated particle

25. STAR Dihadron correlations in Df PRL 91, 072304 Striking final state effects preliminary 20-60% central trigger in-plane trigger out-of-plane

26. Conclusions from the study of jet physics • Substantial suppression of high pT particles in central AuAu collisions -- hot medium effect • No suppression in dAu collisions -- cold medium • Jet quenching -- hard partons lose energy while passing through hot and dense medium (sQGP) That’s all good news. Now, the bad news.

27. h D(z) q A A Conventional approach to hadron production at high pT in heavy-ion collisions at high energy --- follow what’s done in particle physics Framework: hard scattering  fragmentation

28. Anomalies: • proton-to-pion ratio • Cronin effect • Azimuthal anisotropy • Jet structure • Suppression in forward production • Correlations

29. Intermediate pT: anomalous baryon production PRL 91, 172301 (2003) Central Au+Au: baryon/meson yields substantially in excess of expectations from jet fragmentation

30. Rp/π Not possible in fragmentation model: u Exhibit #1 Rp/π  1

31. cm energy cm energy

32. PHENIX and STAR experiments found (2002) Can’t be explained by fragmentation. RHIC data at 200 GeV per NN pair Ratio of central to peripheral collisions: RCP

33. PHENIX PRL 88, 24301(2002) central peripheral

34. Exhibit #2 in pA or dA collisions Cronin Effect Cronin et al, Phys.Rev.D (1975) h q p kT broadening by multiple scattering in the initial state. A p >  Unchallenged for ~30 years. If the medium effect is before fragmentation, then  should be independent of h=  or p

36. v2(p) > v2() at pT > 2.5 GeV/c Exhibit #3 Azimuthal anisotropy v2: coeff. of 2nd harmonic in  distribution

37. forward has more transverse broadening • backward has no broadening Forward-backward asymmetry in d+Au collisions If initial transverse broadening of parton gives hadrons at high pT, then Expects more forward particles at high pT than backward particles

38. Exhibit #4 Backward-forward asymmetry at intermed. pT in d+Au collisions

39. Rapidity dependence of RCP in d+Au collisions BRAHMS PRL 93, 242303(2004) Central more suppressed than peripheral collisions RCP < 1 at =3.2 Interpreted as possible signature of Color Glass Condensate.

40. trigger particle associated particles The distribution of the associated particles should be independent of the medium if fragmentation takes place in vacuum. Exhibit #5 Jet structure Hard parton  jet { (p1) + (p2) + (p3) + ···· }

41. pp Exhibit #5 Jet structure for Au+Au collisions is different from that for p+p collisions Fuqiang Wang (STAR) Quark Matter 2004

42. Resolution: Parton Recombination All anomalies are in the context of the standard procedure: FRAGMENTATION

43. but not for AA collisions. recombination What about strings? • String model may be relevant for pp collisions, • String/fragmentation has no phenomenological support in heavy-ion collisions.

44. Pion Distribution Inclusive distribution of pions in any direction

45. Proton formation: uud distribution usual fragmentation soft component soft semi-hard components (by means of recombination) Pion formation: distribution thermal shower

46. soft TT TS hard SS thermal Pion distribution (log scale) fragmentation Transverse momentum There are many phenomenological successes of this picture.

47. fragmentation thermal  production in AuAu central collision at 200 GeV Hwa & CB Yang, PRC70, 024905 (2004)

48. All in recombination/ coalescence model p/ ratio

49. soft-soft No pT broadening by multiple scattering in the initial state. Medium effect is due to thermal (soft)-shower recombination in the final state. Cronin effect in d+Au collisions pion Hwa & CB Yang, PRL 93, 082302 (2004)

50. STAR data Azimuthal anisotropy Molnar and Voloshin, PRL 91, 092301 (2003). Parton coalescence implies that v2(pT) scales with the number of constituents quark momentum = hadron momentum/n