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

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, T init =340 MeV,  init =25 GeV/fm 3

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

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

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.

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

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

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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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.

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,

16 Jets in RHI Collisions p+p  jet+jet Au+Au  ??? nucleon parton jet Find this……….in this

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

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

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

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

24 Azimuthal dependence of jet production trigger associated particle

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

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)

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

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

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

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

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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.

33 PHENIX PRL 88, 24301(2002) central peripheral

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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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

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

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

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

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.

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

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

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?

44 Inclusive distribution of pions in any direction PionDistribution

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

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

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

48 All in recombination/ coalescence model p/  ratio

49 Cronin effect in d+Au collisions Hwa & CB Yang, PRL 93, (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

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

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

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

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

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.