Berndt Mueller (Duke University) LANL Theory Colloquium 2 June 2005

1. ANEARLY PERFECT INK:The quest for the quark-gluon plasma at the Relativistic Heavy Ion Collider Berndt Mueller (Duke University) LANL Theory Colloquium 2 June 2005

2. STAR The Road to the Quark-Gluon Plasma… …Is Hexagonal and 2.4 Miles Long Insights and Scientific Challenges from the RHIC Experiments

3. The quest for simplicity The equation of state of strongly interacting matter according to lattice QCD • Before the 1975, matter at high energy density was considered a real mess! • QCD predicts that hot matter becomes simple – the QGP (not necessarily weakly interacting!). • Characteristic features:deconfinementandchiral symmetry restoration. Quark-gluon plasma Tc≈ 160 MeV

4. T Critical endpoint ? Quark- Gluon Plasma RHIC Chiral symmetry restored Hadronic matter 1st order line ? Colour superconductor Chiral symmetry broken B Nuclei Neutron stars The QCD phase diagram Tc

5. Space-time picture of a r.h.i.c. Thermal freeze-out Hadronization Equilibration

6. RHIC data gathering RHIC has had runs with: • Au+Au at 200, 130, 62 GeV • Cu+Cu at 200, 62 GeV • d+Au at 200 GeV • p+p at 200, 130 GeV Charged particle tracks from a central Au+Au collision

7. Frequently Asked Questions • How do we know that we have produced equilibratedmatter, not just a huge bunch of particles ? • What makes this matter special ? • How do we measure its properties ? • Which evidence do we have that quarks are deconfined for a brief moment (about 10-23 s) ? • Which evidence do we have for temporary chiral symmetry restoration ? • What do we still need to learn ? • Translation: When can RHIC be closed down ?

8. FAQ #1 How do we know that we produced equilibratedmatter, not just a bunch of particles ? Answer: Particles are thermally distributed and flow collectively !

9. Central Au-Au √s=200 GeV STAR Preliminary Chemical equilibrium • Chemical equilibrium fits work, except where they should not (resonances with large rescattering). RHIC Au+Au @ 200 GeV • Tch = 160  10 MeV • µB = 24  5 MeV

10. Coordinate space: initial asymmetry Momentum space: final asymmetry y Pressure gradient collective flow py px x Elliptic flow Two-particle correlations dN/d(1- 2) 1 + 2v22cos(2[1- 2])

11. FAQ #2 What makes this matter special? Answer: It flows astonishingly smoothly ! “The least viscous non-superfluid ever seen”

12. pQCD: But what is as ? Dimensionless quantity v2 requires low viscosity Relativistic viscous fluid dynamics: Shear viscosity htends to smear out the effects of sharp gradients

13. Viscosity must be ultra-low v2 data comparison with (2D) relativistic hydrodynamics results suggests h/s  0.1 Recent excitement: D. Teaney Quantum lower bound on h/s : h/s = 1/4p (Kovtun, Son, Starinets) Realized in strongly coupled (g1)N = 4 SUSY YM theory, also in QCD ? h/s = 1/4p implies lf≈ (5 T)-1 ≈ 0.3 d QGP(T≈Tc) = sQGP

14. FAQ #3 How do we measure its properties? Answer: With hard QCD probes, such as jets, photons, or heavy quarks

15. High-energy parton loses energy by rescattering in dense, hot medium. q q “Jet quenching” = Parton energy loss Radiative energy loss: L Scattering centers = color charges q q Density of scattering centers g Scattering power of the QCD medium: Range of color force

16. Phenix preliminary Peripheral collisons Central collisons Suppression of fast pions (p0)

17. RHIC data sQGP QGP R. Baier pT = 4.5 – 10 GeV/c Pion gas Cold nuclear matter Larger than expected from perturbation theory ? Energy loss at RHIC (Dainese, Loizides, Paic, hep-ph/0406201) • Data are described by a very large loss parameter for central collisions:

18. I. Vitev (JRO Fellow – LANL) d+Au Au+Au Does QCD perturbation theory work? p0 → What is the appropriate value of QCD coupling as ?

19. FAQ #4 Which evidence do we have that quarks are deconfined for a brief moment (about 10-23 s) ? Answer: Baryons and mesons are formed from independently flowing quarks

20. Suppression Patterns: Baryons vs. Mesons • What makes baryons different from mesons ?

21. Fragmentation Recombination Hadronization Mechanisms This is not coalescence from a dilute medium !

22. Baryons compete with mesons Recombination “wins” … … always for a thermal source Fragmentation still wins for a power law tail

23. R.J. Fries, BM, C. Nonaka, S.A. Bass pQCD spectrum shifted by 2.2 GeV Teff = 350 MeV blue-shifted temperature T = 180 MeV Recombination vs. Fragmentation Baryon enhancement

24. Recombination model suggests that hadronic flow reflects partonic flow (n = number of valence quarks): Provides measurement of partonic v2 ! Hadron v2 reflects quark flow !

25. FAQ #5 Which evidence do we have for temporary chiral symmetry restoration ?

26. (sss) QCD mass disappears (qss) (qqs) Strangeness in Au+Au at RHIC Mass (MeV) Flavor

27. FAQ #6: What do we still need to (or want to) learn ? • Number of degrees of freedom: • via energy density – entropy relation. • Color screening: • via dissolution of heavy quark bound states (J/Y). • Chiral symmetry restoration: • modification of hadron masses via e+e- spectroscopy. • Quantitative determination of transport properties: • viscosity, stopping power, sound velocity, etc. • What exactly is the “s”QGP ?

28. 20% QCD equation of state Challenge: Devise method for determining n from data

29. Alternative method BM & K. Rajagopal, hep-ph/0502174 Eur. J. Phys. C (in print) Eliminate T from e and s : Lower limit on n requires lower limit on s and upper limit on e.

30. Measuring e and s • Entropy is related to produced particle number and is conserved in the expansion of the (nearly) ideal fluid: dN/dy → S→ s = S/V. • Energy density is more difficult to determine: • Energy contained in transverse degrees of freedom is notconserved during hydrodynamical expansion. • Focus in the past has been on obtaining a lower limit on e; here we need an upper limit. • New aspect at RHIC: parton energy loss. dE/dx is telling us something important – but what exactly?

31. Entropy • Two approaches: • Use inferred particle numbers at chemical freeze-out from statistical model fits of hadron yields; • Use measured hadron yields and HBT system size parameters as kinetic freeze-out (Pratt & Pal). • Method 2 is closer to data, but requires more assumptions (HBT radii = geometric radii, isentropic expansion of hadronic gas). • Good news: results agree within errors: • dS/dy = 5100 ± 400 for Au+Au (6% central, 200 GeV/NN)

32. State of the art • 6% central sNN1/2 = 200 GeV Au+Au collisions (t0 = 1 fm/c): • dS/dy = 5100 ± 400→ s = (dS/dy)/(pR2t0) = 33 ± 3 fm-3 • dET/dy = 650 GeV/fm3→ eBj = (dET/dy)/(pR2t0) = 4.2 GeV/fm3 • e > eBj due to longitudinal hydrodynamic work done. PHOBOS estimate is e > 5 GeV/fm3 at t0 = 1 fm/c. Some examples: • e = 5 GeV/fm3→ n = 71 ± 22 • e = 7 GeV/fm3→ n = 26 ± 8 • e = 9 GeV/fm3→ n = 12 ± 4 • Improved determination of e must be an immediate goal.

33. Heavy quarks • Heavy quarks(c, b) provide a hard scale via their mass. Three ways to make use of this: • Color screening of (Q-Qbar) bound states; • Energy loss of “slow” heavy quarks; • D-, B-mesons as probes of collective flow. RHIC program: c-quarks and J/Y; LHC-HI program: b-quarks and . • RHIC data for J/Y are forthcoming (Runs 4 & 5).

34. ˆ ˆ ˆ q q q RHIC data sQGP QGP Pion gas Cold nuclear matter The Baier plot - again • Plotted against e, is the same for a p gas and for a perturbative QGP. • Suggests that is really a measure of the energy density. • Data suggest that may be larger than compatible with Baier plot. • Better calculations are needed. • One approach (Turbide et al.) based on complete LO HTL transport theory, gets RAA right (hep-ph/0502248)

35. Quenched lattice simulations, with analytic continuation to real time, suggest Td 2Tc ! S. Datta et al. (PRD 69, 094507) Karsch et al. J/Y suppression ? Vqq is screened at scale (gT)-1  heavy quark bound states dissolve above someTd. Color singlet free energy Challenge: Compute J/Y spectral function in unquenched QCD

36. QDR/R STAR Data Away-side jet: Q’(DR/R)2 Di-hadron correlations Correlations depend on selected momentum windows

37. v=0.99c “Waking” the sQGP v=0.55c

38. Outlook • The “discovery phase” of RHIC is just hitting its full stride. • Several important observables still waiting to be explored. • Run-4 and -5 data are eagerly anticipated. • More than 109 events will provide many answers and help us to refine the questions. • Many well defined theoretical challenges.