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Hot, Dense, Thermalized Matter at RHIC

Hot, Dense, Thermalized Matter at RHIC

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Hot, Dense, Thermalized Matter at RHIC

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  1. Hot, Dense, Thermalized Matter at RHIC CIPANP Barbara V. Jacak Stony Brook May 21, 2003

  2. outline • Why collide heavy ions? • QCD and the phase transition • the Relativistic Heavy Ion Collider + experiments • What have we learned so far? • Thermalization & pressure build up – early! • (medium-induced) modification of jets • The control experiment: d+Au • Probing deconfinement, Tinit • Conclusions

  3. The Physics of RHIC • Create very high temperature and density matter • as existed ~1 msec after the Big Bang • inter-hadron distances comparable to that in neutron stars • collide heavy ions to achieve maximum volume • Study the hot, dense medium • is thermal equilibrium reached? • transport properties? equation of state? • do the nuclei dissolve into a quark gluon plasma? • Collide Au + Au ions at high energy • s = 200 GeV/nucleon pair, p+p and d+A to compare • Also polarized p+p collisions to study carriers of p’s spin

  4. -Log10 x p-p hep-ex/0304038 Good agreement with NLO pQCD Parton distribution functions Fragmentation functions G-sat. pQCD BFKL, DGLAP Xc(A) Log Q2 Start with pp collisions & pQCD Works! A handle on initial NN interactions In nuclei, also need:

  5. EOS Karsch, Laermann, Peikert ‘99 e/T4 Tc ~ 170 ± 10 MeV (1012 °K) e ~ 3 GeV/fm3 Need QCD in non-perturbative regime Lattice… But, we look for physics beyond simple superposition of NN: Equilibration Collective effects Energy, color transport in dense medium Deconfinement? T/Tc Lattice QCD tells us: Create these conditions to look for new physics

  6. pT Experimental approach Central region has max temperature & density Head-on = “central” collisions  max volume Thermalization? particle spectra, yields Pressure developed? particle/energy flows Medium properties? effects upon probe particles Deconfinement? c and anti-c remain bound as J/Y?

  7. RHIC at Brookhaven National Laboratory RHIC is first dedicated heavy ion collider 10 times the energy previously available!

  8. STAR 4 complementary experiments

  9. Colliding system expands: Energy  to beam direction per unit velocity || to beam pR2 2ct0 Is the energy density high enough? PRL87, 052301 (2001) • e 4.6 GeV/fm3 (130 GeV Au+Au) 5.5 GeV/fm3 (200 GeV Au+Au) YES - well above predicted transition!

  10. history of heavy ion collisions high e, pressure builds up g, g* e+e-, m+m- p, K, p, n, f, L, D, X, W, d, Real and virtual photons from q scattering sensitive to the early stages. Probe also with q and g produced early, & passing through the medium on their way out. Hadrons reflect medium properties when inelastic collisions stop (chemical freeze-out).

  11. Central Au+Au collisions (~ longitudinal velocity) Particle production (lots!) sum particles under the curve, find ~ 5000 charged particles in collision final state (6200 in 200 GeV/A central Au+Au) In initial volume ~ Vnucleus Rescattering should be important!

  12. Hadron spectra – all 4 experiments! BRAHMS: 10% central PHOBOS: 10% PHENIX: 5% STAR: 5% 200 GeV/A Au+Au Protons show velocity boost  to beam. Expect if pressure build-up due to rescattering Data fit well with: Tfo = 110-120 MeV & <t> = 0.5-0.6

  13. Anti-particle/particle ratios vs. rapidity BRAHMS • At y=0 (central coll.) pbar/p = 0.75 ±0.04 K-/K+ = 0.95 ±0.05 p-/p+ = 1.01 ±0.04 • Nearly baryon-free at central y, but not complete transparency • Larger contribution of protons nearer rapidity of the Au beams PRL 90 102301 (Mar. 2003)

  14. Evidence for equilibrated final hadronic state • Simple quark counting: K-/K+ = exp(2ms/T)exp(-2mq/T) = exp(2ms/T)(pbar/p)1/3 = (pbar/p)1/3 • local strangeness conservation K-/K+=(pbar/p)a a = 0.24±0.02 BRAHMS a = 0.20±0.01 for SPS • Good agreement with statistical-thermal model of Beccatini et al. (PRC64 2001) w/T=170 MeV From y=0 to 3 At y=0 PRL 90 102301 Mar. 2003

  15. More evidence for equilibrated final state Observed hadron ratios in agreement with thermal model

  16. Almond shape overlap region in coordinate space Initial state: a barometer called “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

  17. Preliminary STAR STAR Preliminary v2 measured by the experiments 200 GeV: 0.2< pt < 2.0 130 GeV: 0.075< pt < 2.0 200 GeV: 0.150< pt < 2.0 4-part cumulants v2=0.05 200 GeV: Preliminary - Consistent results - At 200 GeV better pronounced decrease of v2 for the most peripheral collisions.

  18. v2 predicted by hydrodynamics Hydro. Calculations Huovinen, P. Kolb, U. Heinz pressure buildup  explosion happens fast  early equilibration ! STAR PRL 86 (2001) 402 Hydro can reproduce magnitude of elliptic flow for p, p. BUT must add QGP to hadronic EOS!! Similar conclusion reached by CM Ko, et al., Kapusta, et al., Bleicher, et al., among others…

  19. schematic view of jet production hadrons leading particle q q hadrons leading particle Physics of hot medium - a unique probe Probe: Jets from hard scattered quarks Observed via fast leading particles or azimuthal correlations between the leading particles • But, before they create jets, the scattered quarks radiate energy (~ GeV/fm) in the colored medium • decreases their momentum (fewer high pT particles) • “kills” jet partner on other side • “jet quenching”

  20. Nuclear Modification of Hadron Spectra? 1. Compare Au+Au to nucleon-nucleon cross sections 2. Compare Au+Au central/peripheral Nuclear Modification Factor: nucleon-nucleon cross section <Nbinary>/sinelp+p AA AA If no “effects”: RAA < 1 in regime of soft physics RAA = 1 at high-pT where hard scattering dominates Suppression: RAA < 1 at high-pT AA

  21. Is Au+Au different? For p0, plot: PHENIX Preliminary Central Peripheral collisions Yes!!

  22. Au-Au s = 200 GeV: high pT suppression! nucl-ex/0304022 Au-Au nucl-ex/0304022

  23. near side away side peripheral central High pT correlations: Au+Au vs p+p STAR PRL 90, 082302 (2003) Peripheral Au + Au Central Au + Au Back-to-back jets are suppressed in central collisions!

  24. Suppression: a final state effect?

  25. Suppression: an initial state effect?

  26. Need a “control”: d+Au collisions

  27. Experiments show NO suppression in d+Au! PHENIX Preliminary p0 STAR Preliminary PHOBOS Preliminary

  28. Do see Cronin effect! “Cronin” enhancement more pronounced in the charged hadron measurement Possibly a larger effect in protons at medium pT

  29. PHENIX Preliminary 0RAA vs. predictions Theoretical predictions: d+Au:I. Vitev, nucl-th/0302002 and private communication. Au+Au:I. Vitev and M. Gyulassy, hep-ph/0208108, to appear in Nucl. Phys. A; M. Gyulassy, P. Levai and I. Vitev, Nucl. Phys. B 594, p. 371 (2001). Initial state: mult. scatt.,shadowing + final state dE/dx (Au+Au) Also: Kopeliovich, et al (PRL88, 232303,2002) predict RpA~1.1 max at pT=2.5 GeV projectile as color dipole anti-shadowing shadowing

  30. n ZDC p Neutron tagged events enhance peripheral collisions Dependence on Ncoll? <Ncoll>=8.5 / 3.6 No evidence for Ncoll dependent Cronin effect

  31. Back-to-back jets observed in d+Au • no normalization to underlying event • “central”: top 20% FTPCE multiplicity

  32. probe rest frame r/ ggg gluon saturation colored glass condensate Mueller, McLerran, Kharzeev, … Wavefunction of low x (very soft) gluons overlap and the self-coupling gluons fuse, thus saturating the density of gluons in the initial state, coupling gets weak  treat as a classical field! 1 J.P Blaizot, A.H. Mueller, Nucl. Phys. B289, 847 (1987). The saturation scale: pT2 ~ sNc 1/p A2/3 dNg/dy (a G(x,pT2)) Predict: suppressed jet cross section and no back-back pairs At RHIC should have saturation effects at higher x than at HERA due to nuclear size.

  33. Vitev & Gyulassy nucl-th/0104066 Combination of hydro. expansion at lower pT with jet quenching at high pT? Medium modifies fragmentation? We see excess protons at high pT Higher than in p+p collisions or fragmentation of gluon jets in e+e- collisions

  34. protons p0, h Do the baryons scale with Nbinary? Baryon yields not suppresed  Ncoll at pT = 2 – 3 GeV/c Looks like hard/soft process interplay… Challenge to initial state explanations (like colored glass)

  35. Other penetrating probes • See that early medium is hot, dense, equilibrated and (probably) induces energy radiation in transiting q,g • What else can we say about it? • J/Y • Test confinement: do bound c + c survive? • Open Charm • Extra heavy quarks from dense gluon gas? • Do the c quarks lose energy like the light quarks? • Dileptons • Tmax from thermal radiation spectrum Need (a lot) more statistics in the data But can take a first look…

  36. J/Y suppression was observed at CERN at s=18 GeV/A NA50 collaboration J/Y yield Fewer J/Y in Pb+Pb than expected! Interpret as color screening of c-cbar by the medium Initial state processes affect J/Y too so interpretation is still debated...

  37. J/Y at RHIC (PHENIX) Energy/Momentum Centrality  Data consistent with: Hadronic comover breakup (Ramona Vogt) w/o QGP Limiting suppression via surface emission (C.Y. Wong) Dissociation + thermal regeneration (R. Rapp)

  38. Total charm quark cross section at RHIC Cross section fits into expected energy dependence No evidence for strong energy loss of charmed quarks…

  39. conclusions • Rapid thermalization! Strong pressure gradients, hydrodynamics works! • EOS beyond hadronic • The hot matter is “sticky” – it absorbs energy • High pT and high mass data look like pQCD + something • Seeenergy loss, disappearance of back-to-back jets • Excess of protons, antiprotons at high pT • Colored glass condensate? • Too early to tell, but stuff is dense, hot, and ~ equilibrated

  40. So, • are we seeing quark gluon plasma? • If it looks like a duck, walks like a duck…. • BUT • Serious conclusion should await • results from the “control” experiment d+Au • (cold matter to measure initial state effects) • theoretical description(s) which hangs together • WE ALSO NEED TO KNOW • Tinitial from thermal photon spectrum • Is there deconfinement-driven J/Y suppression?

  41. A few mysteries…

  42. Hydro describes single + multi-particles But FAILS to reproduce two-particle correlations! • How to increase R without increasing Rout/Rside??? • EOS? • initial T & rprofiles? • emissivity?

  43. Elliptic flow of high momentum particles min. bias v2 p cross p,K (not expected from hydro) v2 Negatives pi-&K-,pbar Positives pi+&K+,p pT (GeV/c) Still flowing at pT = 8 GeV/c? Unlikely! Geometry effect? Hard to reproduce quantitatively!

  44. Why no big energy loss for heavy quarks? no x4 suppression from peripheral to central, as predicted for dE/dx=-0.5GeV/fm! But (we squirm) - Is 40-70% peripheral enough? error bars still big!

  45. Centrality dependence of charm quarks Compare the measurement to (PYTHIA) an event generator tuned for pp collisions… no large suppression as for light quarks! Spectra of electrons from c e + anything

  46. v2 of mesons & baryons to higher pT Au+Au at sNN=200GeV Consistent between PHENIX and STAR pT < 2 GeV/c v2(light) > v2(heavy) Explained by hydro. expansion pT > 2.5 GeV/c v2(light) < v2(heavy) Can it be explained by some combination of geometry + jet quenching? Quark coalescence? S. Voloshin, nucl-ex/0210014 R. Fries et al., nucl-th/0301087 D. Molnar et al. nucl-th/0302014 In-medium fragmentation fn? Model: P.Huovinen, et al., Phys. Lett. B503, 58 (2001)

  47. 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 fit yields vs. mass (grand canonical ensemble) Tch = 175 MeV mB = 51 MeV These are the conditions when hadrons stop interacting T Observed particles “freeze out” at/near the deconfinement boundary!

  48. QCD Phase Transition • Basic (i.e. hard) questions • how does process of quark confinement work? • how nature breaks symmetries  massive particles from ~ massless quarks • transition affects evolution of early universe • latent heat & surface tension  • matter inhomogeneity in evolving universe • equation of state  compression in stellar explosions

  49. Still flowing at pT = 8 GeV/c? Unlikely!! A puzzle at high pT Nu Xu Adler et al., nucl-ex/0206006