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

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

4. 3 Major achievements Hydrodynamical description of HIC Elliptic flow Jet quenching Strongly interacting Quark Gluon Plasma

5. 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

6. 5 Results on single-particle distributions from hydro Kolb & Heinz, QGP3  equ =0.6 fm, T init =340 MeV,  init =25 GeV/fm 3

7. 6 Azimuthal anisotropy Non-central collision  dependence Spatial eccentricityMomentum eccentricity  = 0°  = 90° x y Requires early thermalization to build up pressure for expansion in the x direction

8. 7 Elliptic flow -- v 2 Huovinen, Kolb, Heinz, Ruuskanen, Voloshin Phys. Lett. B 503 58, (2001). Agree with data for p T <2GeV/c possible only if  therm <1 fm/c

9. 8 Flattening of v 2 flattening of spectrum for heavier particles Kolb & Rapp, PRC 2003 Heavier particles have more, and flatter slope. largerflow Good support for hydrodynamics for p T < 2 GeV/c.

10. 9 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) sQGP

11. 10 But hydrodynamics does not work for p T > 2 GeV/c Traditional regions: p T < 2 GeV/c soft p T > 2 GeV/c hard Soft region: macroscopic physics, collective variables. Hard region: microscopic physics, parton variables. Hard scattering of partons -- pQCD +  Jets

12. 11 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. High p T Physics of Nuclear Collisions at High Energy particle

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15. 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 contain quarks We need to know how to interpret the hadron spectra to learn about the quarks and gluons.

16. 15 “Jets” via dihadron azimuthal distributions p+p  dijet trigger: highest p T track  distribution: 2 GeV/c<p T <p T trigger normalize to number of triggers trigger Phys Rev Lett 90, 082302

17. 16 Jets in RHI Collisions p+p  jet+jet (STAR@RHIC) Au+Au  ??? (STAR@RHIC) nucleon parton jet Find this……….in this

18. 17 Inclusive hadron suppression in Au+Au p T (GeV) PHENIX PHOBOS at different centralities

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22. 21 High p T yields in central Au+Au are suppressed D. d’Enterria x5 Factor 5 suppression: huge effect! Binary Collision scaling

23. 22 Is suppression an initial or final state effect? Initial state? gluon saturation Final state? partonic energy loss How to discriminate? Turn off final state  d+Au collisions

24. 23 d+Au yields are not suppressed PHENIX PHOBOS BRAHMS STAR PRL 91, 072302/3/4/5 Hadron suppression in central Au+Au is a final state effect

25. 24 Azimuthal dependence of jet production trigger associated particle

26. 25 Dihadron correlations in  PRL 91, 072304 STAR preliminary 20-60% central trigger in-plane trigger out-of-plane Striking final state effects

27. 26 That’s all good news. Now, the bad news. Conclusions from the study of jet physics Substantial suppression of high p T 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)

28. 27 Conventional approach to hadron production at high p T in heavy-ion collisions at high energy --- follow what’s done in particle physics D(z) h q AA Framework: hard scattering  fragmentation

29. 28 Anomalies: proton-to-pion ratio Cronin effect Azimuthal anisotropy Jet structure Suppression in forward production Correlations

30. 29 Intermediate p T : anomalous baryon production PRL 91, 172301 (2003) Central Au+Au: baryon/meson yields substantially in excess of expectations from jet fragmentation

31. 30 Exhibit #1 R p/π  1 Not possible in fragmentation model: R p/π u

32. 31 cm energy

33. 32 RHIC data at 200 GeV per NN pair Ratio of central to peripheral collisions: R CP PHENIX and STAR experiments found (2002) Can’t be explained by fragmentation.

34. 33 PHENIX PRL 88, 24301(2002) central peripheral

35. 34 k T broadening by multiple scattering in the initial state. Unchallenged for ~30 years. If the medium effect is before fragmentation, then  should be independent of h=  or p Exhibit #2 in pA or dA collisions Cronin Effect Cronin et al, Phys.Rev.D (1975) p q h A  p >  

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37. 36 Azimuthal anisotropy Exhibit #3 v 2 (p) > v 2 (  ) at p T > 2.5 GeV/c v 2 : coeff. of 2nd harmonic in  distribution

38. 37 Forward-backward asymmetry in d+Au collisions Expects more forward particles at high p T than backward particles If initial transverse broadening of parton gives hadrons at high p T, then backward has no broadening forward has more transverse broadening

39. 38 Exhibit #4 Backward-forward asymmetry at intermed. p T in d+Au collisions

40. 39 Rapidity dependence of R CP in d+Au collisions BRAHMS PRL 93, 242303(2004) R CP < 1 at  =3.2 Central more suppressed than peripheral collisions Interpreted as possible signature of Color Glass Condensate.

41. 40 Exhibit #5 Jet structure Hard parton  jet {  (p 1 ) +  (p 2 ) +  (p 3 ) + ···· } trigger particleassociated particles The distribution of the associated particles should be independent of the medium if fragmentation takes place in vacuum.

42. 41 Exhibit #5 Jet structure for Au+Au collisions is different from that for p+p collisions pp Fuqiang Wang (STAR) Quark Matter 2004

43. 42 All anomalies are in the context of the standard procedure: FRAGMENTATION Resolution: Parton Recombination

44. 43 recombination String model may be relevant for pp collisions, String/fragmentation has no phenomenological support in heavy-ion collisions. but not for AA collisions. What about strings?

45. 44 Inclusive distribution of pions in any direction PionDistribution

46. 45 Pion formation:distribution thermal shower soft component soft semi-hard components usual fragmentation (by means of recombination) Proton formation: uud distribution

47. 46 thermal fragmentation softhard TS Pion distribution (log scale) Transverse momentum TT SS There are many phenomenological successes of this picture.

48. 47  production in AuAu central collision at 200 GeV Hwa & CB Yang, PRC70, 024905 (2004) fragmentation thermal

49. 48 All in recombination/ coalescence model p/  ratio

50. 49 Cronin effect in d+Au collisions Hwa & CB Yang, PRL 93, 082302 (2004) No p T broadening by multiple scattering in the initial state. Medium effect is due to thermal (soft)-shower recombination in the final state. soft-soft pion

51. 50 Molnar and Voloshin, PRL 91, 092301 (2003). Parton coalescence implies that v 2 (p T ) scales with the number of constituents STAR data Azimuthal anisotropy quark momentum = hadron momentum/n

52. 51 Hwa, Yang, Fries, PRC 71, 024902 (2005) Forward production in d+Au collisions Underlying physics for hadron production is not changed from backward to forward rapidity. BRAHMS data

53. 52 Chiu & Hwa, nucl-th/0505014 Correlation of particles in a jet

54. 53 Anomalies: proton-to-pion ratio Cronin effect Azimuthal anisotropy Jet structure Suppression in forward production Correlations All understood in the framework of parton recombination

55. 54 Conclusion Hydrodynamics works well at low p T. There is a strongly interacting QGP. Strong suppression of high p T particles in AuAu, but not in dAu collisions. Jets are quenched in hot, dense medium. All phenomena at intermediate p T can be understood in terms of hadronization by parton recombination.