Does RHIC produce a strongly coupled quark gluon plasma?

1. Does RHIC produce a strongly coupled quark gluon plasma? Barbara V. Jacak Stony Brook Dec. 1, 2004

2. outline • Why should we expect a strongly coupled quark gluon plasma? • Collective motion, opacity of the plasma • What do data and lattice say? • Underlying degrees of freedom? • What needs to be done next?

3. Relativistic heavy ion collisions • Create high(est) energy density matter • similar to that existing ~1 msec after the Big Bang • can study only in the lab – relics from Big Bang inaccessible • T ~ 200-400 MeV (~ 2-4 x 1012 K) • e ~ 5-15 GeV/fm3 (~ 1030 J/cm2) • R ~ 10 fm, tlife ~ 10 fm/c (~3 x 10-23 sec) • The goal: characterize the hot, dense medium • expect QCD phase transition to quark gluon plasma • does medium behave as a plasma? coupling weak or strong? • What’s the density, temperature, radiation rate, collision frequency, conductivity, opacity, Debye screening length? • probes: passive (radiation) and those created in the collision

4. QGP energy density • > 1 GeV/fm3 i.e. > 1030 J/cm3 Energy Density in the Universe high energy density: e > 1011 J/m3 P > 1 Mbar I > 3 X 1015W/cm2 Fields > 500 Tesla

5. At high temperature and density: Debye screening by produced color-charges expect transition to free gas of quarks and gluons + +… QCD phase transition Color charge of gluons  self interaction; theory is non-abelian • curious properties at large distance: • confinement of quarks in hadrons

6. Karsch, Laermann, Peikert ‘99 ~15% from ideal gas of weakly interacting quarks & gluons e/T4 T/Tc Tc ~ 170 ± 10 MeV (1012 °K) e ~ 3 GeV/fm3 Proper non-perturbative QCD - lattice required conditions to create quark gluon plasma

7. Guide to expectations • I’ve already tossed around some numbers • Based on QCD coupling value + expected temperature • Look more closely at screening, plasma “coulomb” coupling parameter, energy density

8. Screening by the QGP Follow usual derivation of Debye screening • Now put in QGP scales and assumptions: • Hadrons with radii greater than ~ lD will be dissolved • Origin of J/Y suppression QGP signature Strong interaction

9. Plasma coupling parameter • High quark & gluon density • estimate G = <PE>/<KE> • using QCD coupling strength • <PE>=g2/d d ~1/(41/3T) • <KE> ~ 3T • g2 ~ 4-6 (value runs with T) • G ~ g2 (41/3T)/ 3T so plasma parameter G ~ 3 • So the quark gluon plasma is a strongly coupled plasma • As in warm, dense plasma at lower (but still high) T • But strong interaction rather than electromagnetic • G in this range implies liquid properties of the plasma

10. 8 gluons, 2 spins; 2 quark flavors, anti-quarks, 2 spins, 3 colors 37 (!) Estimate energy density Normal nuclear matter: e ~ 0.2 GeV/fm3 ;r ~ 0.17 /fm3 In quark gluon plasma: Energy density for “g” massless d.o.f ~ 12 T4 ~ 2.4 GeV/fm3

11. Hard scattered or heavy q,g probes of plasma formed p, K, p, n, f, L, D, X, W, d, Hadrons reflect (thermal) properties when inelastic collisions stop (chemical freeze-out). g, g* e+e-, m+m- Real and virtual photons emitted as thermal radiation. look for evidence for QGP liquid e, pressure builds up we focus on mid-rapidity (y=0), as it is the CM of colliding system 90° in the lab at collider

12. Almond shape overlap region in coordinate space Collective “elliptic flow” Origin: spatial anisotropy of the system when created, followed by multiple scattering of particles in the evolving system spatial anisotropy  momentum anisotropy v2: 2nd harmonic Fourier coefficient in azimuthal distribution of particles with respect to the reaction plane

13. The data show Particle emission really is azimuthally anisotropic Magnitude of the anisotropy grows with beam energy, then flattens c.m. beam energy

14. Hydro. Calculations Huovinen, P. Kolb, U. Heinz STAR v2 reproduced by hydrodynamics PRL 86 (2001) 402 central • see a large pressure buildup • anisotropy  happens fast while system is deformed • success of hydrodynamics early equilibration ! • ~ 0.6 fm/c • Viscosity very small  a perfect liquid!

15. Kolb, et al Collective motions probes early phase Azimuthal anisotropy Reflects early pressure when overlap is spatially not isotropic Hydrodynamics reproduce magnitude of elliptic flow for p, p. BUT correct mass dependence requires softer than hadronic EOS!!

16. Elliptic flow scales as number of quarks • v2 scales ~ with # of quarks! • evidence that quarks are the particles when the pressure is built up

17. E. Shuryak

18. Lattice QCD shows qq resonant states at T > Tc, also implying high interaction cross sections NOT ideal gas of q,g! Need large s for fast equilibration & large v2 Parton cascade using free q,g scattering cross sections underpredicts pressure Behavior NOT perturbative!

19. The beginning of sQGP: a New QCD Phase Diagram, in which ``zero binding lines” first appeared(ES+I.Zahed hep-ph/030726, PRC) Edward Shuryak on how this can come about T The lines marked RHIC and SPS show the adiabatic cooling paths Chemical potential B

20. Why is hydro description so good ? => near zero binding provides large objects ,maybe large cross sections?(ES+Zahed,03, same) Well, it was shown to work for strongly coupled atoms

21. Unexpected help from stringtheorists, AdS/CFT correspondence • The viscosity/entropy => 1/4 when g2Nc!1,(D.Son et al 2003), as small as at RHIC! • Multiple Coulomb bound states with l» g2Nc(ES+Zahed, PRD 04)

22. Another kind of plasma probe at RHIC • A favorite probe in EM plasma studies • Hard x-rays • Wavelength short compared to characteristic wavelength of the medium • e.g. a warm, dense plasma at ~1 keV • QCD DOES have an analog! • Hard scattered q, g from initial N-N collisions • 5-10 GeV colored probe with short wavelength • Rate and spectrum calculable with pQCD • look for effects of the medium as it transits • Start by benchmarking the probe!

23. p-p PRL 91 (2003) 241803 Good agreement with NLO pQCD Parton distribution functions Fragmentation functions Start simple p+p collisions QCD works for high p transfer processes!  Have a handle on initial NN interactions by scattering of q, g inside N We also need: p0 These are measured, so known

24. ( pQCD x Ncoll) / background Vogelsang/CTEQ6 ( pQCD x Ncoll) / (background x Ncoll) [w/ the real suppression] [if there were no suppression] pQCD in Au+Au? direct photons At high pT, it also works! gTOT/gp0 Au+Au 200 GeV/A: 10% most central collisions Preliminary pT (GeV/c) []measured / []background = measured/background

25. schematic view of jet production hadrons leading particle q q hadrons leading particle AA AA AA nucleon-nucleon cross section <Nbinary>/sinelp+p “external” probes of the medium Hard scattering of q,g early. Observe fast leading particles, back-back correlations Before creating hadron jets, scattered quarks induced to radiate energy (~ GeV/fm) by the colored medium -> jet quenching

26. Au-Au s = 200 GeV: high pT suppressed! PRL91, 072301(2003) Proportional to the density of scattering centers (gluons)  dNg/dy ~1000

27. near side away side Medium is opaque! peripheral central look for the jet on the other side STAR PRL 90, 082302 (2003) Peripheral Au + Au Central Au + Au

28. Suppression: a final state effect? Hot, dense medium causes Such a medium is absent in d+Au collisions! But Au provides all initial state effects of q, g wavefunction inside a Au nucleus d+Au is the “control” experiment

29. Centrality Dependence Au + Au Experiment d + Au Control PHENIX preliminary • Dramatically different and opposite centrality evolution of AuAu experiment from dAu control. • Jet Suppression is clearly a final state effect.

30. leading particle suppressed hadrons Pedestal&flow subtracted q q So this is the right picture for Au+Au ? Are back-to-back jets there in d+Au? Yes!

31. Property probed: density Induced gluon brehmsstrahlung pQCD calculation depends on number of scatterings Agreement with data: initial gluon density dNg/dy ~ 1100e ~ 15 GeV/fm3 hydro initial state same dAu d-Au Lowest energy radiation sensitive to infrared cutoff. Au-Au

32. Energy  to beam direction per unit velocity || to beam pR2 2ct0 Experimental look at energy density Colliding system expands: E • e 5.5 GeV/fm3 (200 GeV Au+Au) well above predicted transition! value is lower limit: longitudinal expansion rate, formation time overestimated

33. And expect hard-soft recombination C.M. Ko et al, Hwa & Yang PRC68, 034904, 2003 PRC67, 034902, 2003 nucl-th/0401001 & 0403072 Study jet fragmentation to probe medium properties Radiated gluons are collinear (inside jet cone) Can also expect a jet “wake” effect, medium particles “kicked” alongside the jet by energy they absorb Fries, Bass & Mueller nucl-th/0407102

34. Correlations of jet fragments with flowing medium Armesto, Salgado & Wiedemann, hep-ph/0405301 Both consistent with features in data with modest jet fragment energy or EVEN MORE complicated couplings • Edward’s conic flow: a pressure wave or “super wake” i.e. medium response to the energy deposited by jets Does jet fragmentation have a meaning in presence of medium? Mechanisms mix up medium & radiated partons New tool to see conductivity & correlations in medium at ~1 fm/c??

35. STAR Preliminary Near-Side ratio p+p 40-80% Au+Au 0-5% Au+Au AuAu/pp More partners on same side nucl-ex/0408007 In 55° Df and small Dh In large Dh

36. Current knowledge on properties • Extract from models, constrain by data Equation of state not hadronic Early degrees of freedom apparently partonic Cross section of resonances at high T? Conductivity?

37. Other strongly coupled plasmas • Inside white dwarf and neutron stars (n star core may even contain QGP) • In ionized gases subjected to very high pressures or magnetic fields • Dusty plasmas in interplanetary space & planetary rings • Solids blasted by a laser • We would like to know: • How do these plasmas transport energy? • How quickly can they equilibrate? • What is their viscosity? G >10 can even be crystalline! • How much are the charges screened? • Is there evidence of plasma instabilities at RHIC? • Can we detect waves in this new kind of plasma? novel plasma of strong interaction

38. Conclusions • Answer: yes! • The adventure will be to figure out its properties • Use medium coupling to jet fragmentation to study energy transport (conductivity) • Heavy quark bound states to determine screening length • Lepton pairs and soft photons from thermal radiation to probe temperature Does RHIC produce a strongly coupled quark gluon plasma?

39. How to get there? • Experimental side – upgrade facility (~2009-2015) • increase RHIC luminosity by ~40 • by electron cooling of heavy ion beams • Capabilities of large detectors (2 steps between now & 2015) • technology for rare features in high muliplicity events • secondary decay vertices • background rejection • triggering, readout capabilities • data analysis infrastructure (already write 0.5 pB/year) • Theory progress (over next 5-10 years) • Large scale computing resources • lattice QCD, hydrodynamic & transport simulations • Ideas & people to develop new approaches

40. Scientific objectives: • Heavy quark probes • High mass  high E to produce (made at 1st step in collision) • Screening properties via (c-anticharm) bound states • Do the heavy quarks thermalize? Lose energy? • higher luminosity sufficient statistics of rare, heavy quarks • lattice calculation under relevant conditions • Initial quark gluon plasma temperature • detector upgrade for background rejection • lattice, hydro simulations (with relevant r, coupling) • Characterize this new kind of plasma • radiation rate, conductivity, collision frequency • Need rare probes, including tagged jets • detector & accelerator improvements; simulations • Consistent theoretical picture of • quark gluon plasma, heavy ion collision to connect with data • Need large scale computational resources for numerical simulation

41. Theoretical tools • Large scale computations of fundamental properties • Solve QCD on the lattice • High T & dynamical quarks: need large computers • Simulations • Hydrodynamics (2D mostly, 3D developing) • pressure, viscosity, energy gradients, equation of state • Cascade, non-equilibrium transport models • pre-equilibrium stage, thermalization approach • parton cascade for early phase • hadron cascade for final state interactions • QCD inspired models • Hadronization (when T falls below Tc) • not understood; collective effects must survive it • Constrain with data to extract plasma properties

42. What’s currently known about properties? • Pressure built up very rapidly during collision • observed collective flow • viscosity small (known from hydro. calculation) • interaction s large • Quasiparticles probably exist above Tc • may experience binding, but not as normal hadron • not color neutral as at T<Tc.

43. Predicted signatures of QGP • Debye screening of color charges (Matsui & Satz) • Suppresses rate of c-c bound state (J/Y) • Strangeness enhancement (Rafelski & Mueller) • Tplasma ~ masss-quark • Enhances strange hadron yields (e.g. K/p increased) • Jet quenching (Gyulassy & Wang) • Dense medium induces coherent gluon radiation • Decreased yield of jets of hadrons from hard scattered quarks or gluons • But real goal is to determine the properties of the plasma

44. 0-20% most central Ncoll=779 40-90% most central Ncoll=45 20-40% most central Ncoll=296 Lattice predictions for heavy quarks NB: need temperature too! Color screening?

45. Hydro. Calculations Huovinen, P. Kolb, U. Heinz Hydrodynamics assumes early equilibration Initial energy density is input Equation of state from lattice QCD Solve equations of motion v2 reproduced by hydrodynamics • see large pressure buildup • anisotropy  happens fast •  early equilibration ! STAR PRL 86 (2001) 402 central

46. More energetic baryons in Au+Au PRL 91 (2003) 172301 • p/ ~1 at high pT • in central collisions • > p+p, d+Au > peripheral Au+Au >jets in e+e- • collisions

47. R. Fries, et al pQCD spectrum shifted by 2.2 GeV Teff = 350 MeV Are extras from the thermal medium? Hydrodynamic expansion common  velocity boost heavier particles boosted to higher pT enhances mid pT hadrons baryons especially Do the hadrons get boosted? or Coalescence of boosted quarks into hadrons?

48. equilibrated final state, including s quarks Hadron yields in agreement with Grand Canonical ensemble T(chemical freeze-out) ~ 175 MeV (multi)strange hadrons enhanced over p+p, but QGP hypothesis not unique explanation

49. early universe 250 RHIC 200 quark-gluon plasma 150 SPS Lattice QCD AGS deconfinement chiral restauration thermal freeze-out 100 SIS hadron gas 50 neutron stars atomic nuclei 0 0 200 400 600 800 1000 1200 Baryonic Potential B [MeV] Locate RHIC on phase diagram From fit of yields vs. mass (grand canonical ensemble): Tch = 176 MeV mB = 41 MeV These are the conditions when hadrons stop interacting T Observed particles “freeze out” at/near the deconfinement boundary!

50. Protons are flatter  velocity boost  to beam Result of pressure built up p, K, p, pbar spectra indicate pressure Look at “transverse mass” mT2 = pT2 + m02 — is distribution e-E/T? i.e. Boltzmann distribution from thermalized gas? yes !