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On the trail of the quark-gluon plasma at RHIC. Gordon Conference Barbara V. Jacak Stony Brook July 20, 2003. outline. Why collide heavy ions? QCD and the phase transition the Relativistic Heavy Ion Collider + experiments What have we learned so far?

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On the trail of the quark-gluon plasma at RHIC


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    1. On the trail of the quark-gluon plasma at RHIC Gordon Conference Barbara V. Jacak Stony Brook July 20, 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 • Medium effects on fragmentation function? • 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. + +… Quantum ChromoDynamics • Strong interaction field theory : colored quarks exchange gluons • Parallels QED but gluons have color charge • unlike E&M where g are uncharged •  they interact among themselves (i.e. theory is non-abelian): curious properties short distance: force is weak (probe w/ high Q2, Calculate with perturbation theory) large distance: force is strong (probe w/ low Q2, calculations must be non-perturbative) leads to confinement High temperature: force becomes screened by produced color-charges

    5. 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

    6. p-p hep-ex/0304038 Good agreement with NLO pQCD Parton distribution functions Fragmentation functions G-sat. pQCD >2 BFKL, DGLAP Xc(A) RHIC s = 200 GeV: start with pQCD & pp collisions Works! A handle on initial NN interactions also need: p0 -Log10 x Log Q2

    7. EOS Karsch, Laermann, Peikert ‘99 e/T4 Tc ~ 170 ± 10 MeV (1012 °K) e ~ 3 GeV/fm3 In A+A: 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 Physics is soft Lattice QCD says: Create these conditions to look for new physics

    8. 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?

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

    10. STAR 4 complementary experiments

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

    12. 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).

    13. 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!

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

    15. Evidence for equilibrated final hadronic state BRAHMS • 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

    16. More evidence for equilibrated final state Observed hadron ratios in agreement with thermal ratios! T(chemical freeze-out) ~ 175 MeV

    17. Almond shape overlap region in coordinate space Early 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

    18. 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.

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

    20. schematic view of jet production hadrons leading particle q q hadrons leading particle a unique probe for physics of hot medium 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”

    21. 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 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

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

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

    24. Suppression: a final state effect? Hadron gas • Hadronic absorption of fragments: • Gallmeister, et al. PRC67,044905(2003) • Fragments formed inside hadronic medium • Parton recombination (up to moderate pT) • Fries, Muller, Nonaka, Bass nucl-th/0301078 • Lin & Ko, PRL89,202302(2002) • Energy loss of partons in dense matter • Gyulassy, Wang, Vitev, Baier, Wiedemann… Not technically quite a Final state effect… Absent in d+Au collisions! d+Au is the “control” experiment

    25. probe rest frame r/ ggg Suppression: an initial state effect? • Gluon Saturation • (color glass condensate) Wavefunction of low x gluons overlap; the self-coupling gluons fuse, saturating the density of gluons in the initial state.(gets Nch right!) • Multiple elastic scatterings (Cronin effect) Wang, Kopeliovich, Levai, Accardi • Nuclear shadowing Gribov, Levin, Ryshkin, Mueller, Qiu, Kharzeev, McLerran, Venugopalan, Balitsky, Kovchegov, Kovner, Iancu … RdAu~ 0.5 D.Kharzeev et al., hep-ph/0210033 Broaden pT :

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

    27. Do see Cronin effect! (h++h-)/2 “Cronin” enhancement more pronounced in the charged hadron measurement Possibly larger effect in protons at mid pT Implication of RdAu? RHIC at too high x for gluon saturation… p0

    28. 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

    29. PHENIX Preliminary results, consistent with PHOBOS data in submitted paper Centrality Dependence Au + Au Experiment d + Au Control • Dramatically different and opposite centrality evolution of AuAu experiment from dAu control. • Jet Suppression is clearly a final state effect.

    30. Back-to-back jets observed in d+Au STAR ST • jet pair production • also looks • independent of Ncoll • observe no (big) suppression in back-to back jets! • probably some jet broadening due to initial multiple scattering… PHENIX preliminary

    31. Vitev&Gyulassy nucl-th/0104066 Hydro. expansion at low pT + jet quenching at high pT? Medium modified fragmentation function? Particle mix at RHIC is different! p/ ~1 at high pT in central collisions Higher than in p+p or jets in e+e- collisions

    32. Do the baryons scale with Ncoll? Au+Au Baryon yields not suppresed  Ncoll at pT = 2 – 4 GeV/c hard/soft process interplay? Quark recombination? Medium modification of fragmentation function?

    33. To help sort it out, study initial state effects d+Au PHENIX preliminary high pT What is initial state multiple scattering mechanism? low pT =n How is production of mesons and baryons affected? fragmentation function…

    34. Deconfinement? Does colored medium screen c+cbar? Look at J/Y 40-90% most central Ncoll=45 20-40% most central Ncoll=296 0-20% most central Ncoll=779 R.L. Thews, M. Schroedter, J. Rafelski Phys. Rev. C63 054905 (2001): Plasma coalesence model for T=400MeV and ycharm=1.0,2.0, 3.0 and 4.0. L. Grandchamp, R. RappNucl. Phys. A&09, 415 (2002) and Phys. Lett. B 523, 50 (2001): Nuclear Absorption+ absoption in a high temperature quark gluon plasma A. Andronic et. Al. Nucl-th/0303036 Proton Can’t tell yet, but don’t see EXTRA (thermal) J/Y

    35. conclusions • Rapid equilibration! • Strong pressure gradients, hydrodynamics works • Constituent scattering cross section is very large • EOS is not hadronic • The hot matter is “sticky” – it absorbs energy • Seeenergy loss, disappearance of back-to-back jets • d+Au data says: final state, not initial state effect • So, the stuff is dense, hot, ~ equilibrated AND NEW! • OK, why not announce QGP discovery? • J/Y suppression or not? Next run • Tinitial? direct photon analysis underway by PHENIX • Properties not as expect for plasma – looks like gluon liquid

    36. A few mysteries…

    37. 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?

    38. Elliptic flow of high momentum particles baryons cross mesons (not expected from hydro) v2 pT (GeV/c) Still flowing at pT = 8 GeV/c? Geometry? v2 a bit too big… Mix of flow + jets???

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

    40. 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

    41. 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!

    42. n ZDC p Neutron tagged events enhance peripheral collisions is d+Au same as p+Au? <Ncoll> = 5.0 / 3.6 Could be Ncoll dependence d+Au looks very similar to p+Au

    43. Run 2001/2002 Au-Au 200 GeV: Particle Composition at high pT (h++h-)/2p0 ~ 50% greater in central than peripheral at mid pT similar again for pT>5 GeV/c Central Peripheral

    44. 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!

    45. 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)

    46. 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

    47. 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...

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

    49. Vitev: they can get v2 right • There is a quantitative difference • Calculations/fits with flat • or continuously growing Check against high-pT data (200 AGeV) b=7 fm b~7 fm C. Adler et al. [STAR Collab.], arXiv: nucl-ex/0206006 Same for 0-50% • The decrease with pT is now • supported by data • For minimum bias this rate is • slightly slower K. Filimonov [STAR Collab.], arXiv: nucl-ex/0210027 See: N.Borghini, P.Dinh, J-Y.Ollitrault, Phys.Rev. C 64 (2001)