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Low and Intermediate P t Probes of the Perfect Liquid at RHIC

Low and Intermediate P t Probes of the Perfect Liquid at RHIC. James C Dunlop Brookhaven National Laboratory. Outline. “Perfect Liquid”: hydrodynamic system with extremely low viscosity Experimental measures: v 2 , spectra Theoretical issues and comparison to data

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Low and Intermediate P t Probes of the Perfect Liquid at RHIC

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  1. Low and Intermediate Pt Probes of the Perfect Liquid at RHIC James C Dunlop Brookhaven National Laboratory James Dunlop Aspen Feb. 2006

  2. Outline • “Perfect Liquid”: hydrodynamic system with extremely low viscosity • Experimental measures: v2, spectra • Theoretical issues and comparison to data • Intermediate pT: degrees of freedom at hadronization • Coalescence/recombination of “constituent” quarks (including charm?) • Throughout the talk I will attempt to highlight where quantitative progress can be made James Dunlop Aspen Feb. 2006

  3. RHIC Implementation PHOBOS BRAHMS &PP2PP RHIC PHENIX 1.2 km STAR • Flexibility is key to understanding complicated systems • Polarized protons, sqrt(s) = 50-500 GeV • Nuclei from d to Au, sqrt(sNN) = 20-200 GeV • Physics runs to date • Au+Au @20,62,130,200 GeV • Cu+Cu @22,62,200 GeV • d+Au @ 200 GeV • Polarized p+p @200 GeV James Dunlop Aspen Feb. 2006

  4. RHIC Experiments Four experiments, two large, two small: STAR: Large acceptance (Df = 2p, Dh = 2-6) PHENIX: Electron/muon identification, high rate trigger, limited acceptance (Df = p, Dh = 0.5 (central arm) PHOBOS: Tabletop: limited tracking acceptance, largest multiplicity acceptance of all experiments BRAHMS: Forward tracking in classical spectrometer James Dunlop Aspen Feb. 2006

  5. Collective Behavior: Azimuthal Anisotropy v2 coordinate-space-anisotropy  momentum-space-anisotropy y py px x Pressure converts initial coordinate-space Anisotropy into final momentum-space anisotropy James Dunlop Aspen Feb. 2006

  6. Time evolution in Ideal Hydrodynamics • Elliptic Flow reduces spatial anisotropy -> shuts itself off • Sensitive to EARLY TIMES James Dunlop Aspen Feb. 2006

  7. Analogy to Ultracold Atoms Extremely cold system at T=10 nK or 10^(-12) eV can produce micro-bang Elliptic flow with ultracold trapped Li6 atoms, a=> infinity regime The system is extremely dilute, but can be put into a hydro regime, with an elliptic flow, if it is specially tuned into a strong coupling regime via the so called Feshbach resonance Analogy pointed out by Shuryak James Dunlop Aspen Feb. 2006

  8. v2: Excitation Function STAR, Nucl. Phys. A 757 (2005) 102 • Excitation function of v2, integrated over pT, vs. energy and Nch density • At RHIC: for the first time, ideal hydrodynamics describes the data U+U LHC James Dunlop Aspen Feb. 2006

  9. v2 vs. Ideal Hydrodynamics Ideal hydrodynamics reproduces v2 relatively well Below pT~2 GeV, matches v2 and spectra to ~20-30% Appealing picture: Nearly perfect fluid with local thermal equilibrium established at <~1 fm with a soft equation of state containing a QGP stage Hydro calculations: Kolb, Heinz and Huovinen STAR, Nucl. Phys. A 757 (2005) 102 James Dunlop Aspen Feb. 2006

  10. In Au+Au Collisions: Interactions create Flow Molnar, Gyulassy 2001 • In analogy to ultracold atoms (cascade model of Au+Au collisions) • Small (such as pQCD) cross-sections: small amounts of v2 • Large cross-sections: necessary in cascade model to match data • This was NOT expected: instead more weakly interacting plasma James Dunlop Aspen Feb. 2006

  11. How ideal is ideal? D. Teaney, Phys. Rev. C 68, 034913 (2003) First attempt at viscous effects: Large effect on v2 Conclusion: viscosity must be extremely small (near quantum lower bound?) James Dunlop Aspen Feb. 2006

  12. “Perfect Liquid” • Large values of v2, combined with the need for low viscosity (and therefore strong coupling), led to the announcement last year that “RHIC Scientists Serve Up the Perfect Liquid” • http://www.bnl.gov/bnlweb/pubaf/pr/PR_display.asp?prID=05-38 James Dunlop Aspen Feb. 2006

  13. Detailed comparison of results to hydro calculations PHENIX, Nucl. Phys. A 757 (2005) 184 • Model parameters derived from v2 and spectra: how well? James Dunlop Aspen Feb. 2006

  14. Score board: status of hydrodynamic models PHENIX, Nucl. Phys. A 757 (2005) 184 • Hadronic + QGP hydro reproduces features of v2(pT) of p, K, p • Require early thermalization (ttherm<1fm/c) + high einit > 10 GeV/fm3 • Detailed discrepancies between models and with experiment Best reproduction: Teaney, combining late-stage hadronic rescattering with ideal hydrodynamics at the early stage • Theoretical progress needed on complete calculation to be quantitative: viscous, 3D hydro + hadronic final state James Dunlop Aspen Feb. 2006

  15. A note on v2: “non-flow” jet parton nucleon nucleon If one naively measures v2 in p+p collisions, how big a signal do you see? (Hint: it’s not 0) p+p jet+jet (STAR@RHIC) STAR, Phys. Rev. Lett. 93(2004) 252301 James Dunlop Aspen Feb. 2006

  16. Experimental limitations on v2 STAR, Phys. Rev. C 72(2005) 014904 • In Au+Au collisions, methods in determining v2 disagree by 10-20% • Possible event-by-event fluctuations in initial geometry affect measures differently: <v24>1/4 != <v22>1/2 != <v2> (Miller, Snellings, nucl-ex/0312008) • Problem more serious in lighter systems such as Cu+Cu • Active experimental program to reduce uncertainties from method James Dunlop Aspen Feb. 2006

  17. The Future: Charm v2 PHENIX, nucl-ex/0510008 and PRC 72 (2005) 024901 • Indications from single-electron v2 that charm v2 is sizable • Implies strong rescattering of charm in the medium • Much stronger than perturbative estimates (Moore & Teaney, hep-ph/0412346) • May even indicate persistence of charmed bound states in plasma (van Hees, Greco, Rapp, hep-ph/0601166) • Active upgrade program for more definitive measurements • Especially important: direct D measurements in hydrodynamic pT range James Dunlop Aspen Feb. 2006

  18. Breakdown of hydrodynamics: v2 vs. pT Large values indicate strong sensitivity to the system geometry for production at all measured pT v2 at intermediate pT is grouped by quark number Intermediate pT PRL 92 (2004) 052302; PRL 91 (2003) 182301 James Dunlop Aspen Feb. 2006

  19. Scaling of v2 with Number of Constituent Quarks Scale pT and v2 with number of constituent quarks (2 for mesons, 3 for baryons) Low pT: scaling fails (hydrodynamics) Intermediate pT: works rather well (to 5-10%) STAR Preliminary (M.Oldenburg, QM2005) James Dunlop Aspen Feb. 2006

  20. Baryon enhancement Au+Au 0-10% Au+Au 0-10% p+p p+p • Large enhancement in baryon/meson ratios in central Au+Au collisions • Maximum at pT~3 GeV/c, after which approach towards p+p • Indication of dominant non-fragmentation contribution • For pT > 6 GeV, contribution no longer dominant? L/K0s Au+Au 0-5% p+p STAR, nucl-ex/0510073 James Dunlop Aspen Feb. 2006

  21. Intermediate pT: hints of relevant degrees of freedom Clear separation into two classes: baryons and mesons Apparent scaling with number of constituent quarks in final-state hadron Explained currently by recombination/coalescence of constituent quarks at hadronization If better established, direct evidence of the degrees of freedom relevant at hadronization, and the existence of collective flow at the constituent quark level v2/nq STAR, Nucl. Phys. A 757 (2005) 102 and PRL 95 (2005) 123201 James Dunlop Aspen Feb. 2006

  22. Conclusion • Collective flow measures large at RHIC • Hydrodynamics at low viscosity: “Perfect Fluid” • Extensive experimental and theoretical work remaining • Quantify departures: viscous, 3D hydro + hadronic stage • Extend measurements: U+U, LHC; charm • Intermediate pT: hints at relevant degrees of freedom • Dressed flowing “partons”, combining into hadrons • Dominant pT regime limited to <~ 6 GeV/c James Dunlop Aspen Feb. 2006

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