low and intermediate p t probes of the perfect liquid at rhic

James Dunlop Aspen Feb. 2006 1

Low and Intermediate Pt Probes of the Perfect Liquid at RHIC

James C DunlopBrookhaven National Laboratory


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


RHIC Implementation

• 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

PHENIXBRAHMS &PP2PPPHOBOS

STAR 1.2 kmRHIC


RHIC Experiments

Four experiments, two large, two small:

STAR: Large acceptance (PHENIX: Electron/muon identification, high rate trigger, limited acceptance (central arm)

PHOBOS: Tabletop: limited tracking acceptance, largest multiplicity acceptance of all experimentsBRAHMS: Forward tracking in classical spectrometer


Collective Behavior: Azimuthal Anisotropy v2

)(tan,2cos 1222

22

x

y

p

pv

xy

xy

y

x

py

px

coordinate-space-anisotropy momentum-space-anisotropy

Pressure converts initial coordinate-space Anisotropy into final momentum-space anisotropy


Time evolution in Ideal Hydrodynamics

• Elliptic Flow reduces spatial anisotropy -> shuts itself off

• Sensitive to EARLY TIMES


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

Extremely cold system at T=10 nK or 10^(-12) eV can produce micro-bang

Analogy to Ultracold Atoms

Analogy pointed out by Shuryak


v2: Excitation Function

• Excitation function of v2, integrated over pT, vs. energy and Nch density

• At RHIC: for the first time, ideal hydrodynamics describes the data

STAR, Nucl. Phys. A 757 (2005) 102

U+ULHC


Hydro calculations: Kolb, Heinz and Huovinen

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

STAR, Nucl. Phys. A 757 (2005) 102


In Au+Au Collisions: Interactions create Flow

• 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

Molnar, Gyulassy 2001


How ideal is ideal?

First attempt at viscous effects: Large effect on v2

Conclusion: viscosity must be extremely small (near quantum lower bound?)

D. Teaney, Phys. Rev. C 68, 034913 (2003)


“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


Detailed comparison of results to hydro calculations

• Model parameters derived from v2 and spectra: how well?

PHENIX, Nucl. Phys. A 757 (2005) 184


Score board: status of hydrodynamic models

• Hadronic + QGP hydro reproduces features of v2(pT) of , K, p• Require early thermalization (therm<1fm/c) + high init > 10 GeV/fm3

• Detailed discrepancies between models and with experimentBest 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

PHENIX, Nucl. Phys. A 757 (2005) 184


A note on v2: “non-flow”

p+p jet+jet (STAR@RHIC)

nucleon nucleonparton

jet

If one naively measures v2 inp+p collisions, how big a

signal do you see?(Hint: it’s not 0)

STAR, Phys. Rev. Lett. 93(2004) 252301


Experimental limitations on v2

• 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 != <v2

2>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

STAR, Phys. Rev. C 72(2005) 014904


The Future: Charm v2

• 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

PHENIX, nucl-ex/0510008 and PRC 72 (2005) 024901


Breakdown of hydrodynamics: v2 vs. pT

Large values indicate strong sensitivity to the system geometry Large values indicate strong sensitivity to the system geometry for production at all measured pfor production at all measured pTT

vv22 at intermediate p at intermediate pTT is grouped by quark number is grouped by quark number

Intermediate pT

PR

L 92

(20

04

) 05

23

02

; PR

L 91

(20

03

) 18

23

01


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)


Baryon enhancement

• 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? STAR, nucl-ex/0510073

/K

0 s

Au+Au 0-10%

p+p

Au+Au 0-10%

p+p

Au+Au 0-5%

p+p


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

v 2/n q

STAR, Nucl. Phys. A 757 (2005) 102 and PRL 95 (2005) 123201


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

low and intermediate p t probes of the perfect liquid at rhic

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