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QCD and Heavy Ions: an overview

XXXIX Rencontres de Moriond, March 28 - April 4 2004. QCD and Heavy Ions: an overview. D. Kharzeev BNL. Outline. QCD of strong color fields: parton saturation and Color Glass Condensate Manifestations of CGC at RHIC:. hadron multiplicities high p T suppression at forward rapidity.

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QCD and Heavy Ions: an overview

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  1. XXXIX Rencontres de Moriond, March 28 - April 4 2004 QCD and Heavy Ions:an overview D. Kharzeev BNL

  2. Outline • QCD of strong color fields: parton saturation and Color Glass Condensate • Manifestations of CGC at RHIC: • hadron multiplicities • high pT suppression at forward rapidity • From CGC to Quark-Gluon Plasma • Future tests: RHIC, LHC, eRHIC

  3. QCD and the classical limit QCD = Quark Model + Gauge Invariance For , .i Classical dynamics applies when the action is large: => Need weak coupling and strong fields

  4. Asymptotic freedom and the classical limit of QCD Classical limit S>>1 requires weak coupling and strong fields; Large distances: strong fields but large coupling… Is there a place for classical methods?

  5. Parton saturation and the classical limit of QCD At small Bjorken x, hard processes develop over .large longitudinal distances All partons contribute coherently => at sufficiently small x and/or .large A strong fields, weak coupling!

  6. The phase diagram of high energy QCD … no numbers yet, but they will follow

  7. Non-linear effects in the QCD evolution QCD evolution equations (DGLAP, BFKL) predict an increase of parton densities at small Bjorken x, in agreement with experiment; at “sufficiently small” x and/or large A non-linear effects must become important (GLR; MQ; JIMWLK; ILM). At large , equations can be written in a closed form - BK (dipole scatterings are assumed to be independent - similar to the mean field approximation in statistical physics) but correlations may be important - see e.g. E.Iancu, A.H.Mueller, hep-ph/0309276; A.H.Mueller, A.I.Shoshi, hep-ph/0402193

  8. CGC and total multiplicities in Au-Au CGC predicts very simple dependence of multiplicity .on atomic number A / Npart: Almost like in “wounded nucleon” and string-based models; Agrees unexpectedly well with “soft + hard” parameterizations (note: total multiplicities are dominated by small transverse momenta => classical dynamics applicable)

  9. Parton interactions at RHIC .are coherent ! dNch/dh @ h=0 vs Energy

  10. Centrality dependence of hadron multiplicity => Talk by D. Hofman

  11. Centrality dependence at different energies

  12. Initial state parton saturation? QM2002: nucl-ex/0212009 200 GeV 130 GeV Preliminary 19.6 GeV Kharzeev, Levin, Nardi, hep-ph/0111315 l~0.25 from fits to HERA data: xG(x)~x-l Describes energy dependence correctly!

  13. Parton Saturation Describes Au + Au Color Glass Condensate describes the Au-Au data Kharzeev & Levin, Phys. Lett. B523 (2001) 79 PHOBOS Coll., R. Noucier Au + Au at 130 GeV • We need a simpler system such as d + Au in order to understand a complex system Au + Au • The results of d+Au are crucial for testing the saturation approach

  14. D-Au multiplicities Data from BRAHMS and PHOBOS Collaborations DK, E.Levin and M.Nardi, Nucl.Phys.A730(2004)448, + erratum, hep-ph/0212316

  15. From CGC to Quark Gluon Plasma:approach to thermalization The data on azimuthal anisotropy of hadron production indicate strong final state interactions at short time scales U.Heinz, E.Shuryak, … => Talk by R.Snellings Problem: if interactions are perturbative, interaction probabilities are small - suppressed by powers of if interactions are non-perturbative, characteristic re-interaction time is about 1 fm Possible solution: CGC sets a fast time scale (~ 0.2 fm) and classical fields re-interact with ~ O(1) probability (almost) isentropic evolution => ideal hydrodynamics is reasonable R.Baier, A.H.Mueller, D. Schiff, D.T. Son, PLB502(2001)51; 539(2002)46; …

  16. From CGC to Quark Gluon Plasma L. McLerran, T. Ludlam, Physics Today, October 2003

  17. Signatures of the Quark Gluon Plasma • O jet quenching • suppression of heavy quarkonia • collective flow • fluctuations and correlations • dilepton and photon enhancement • particle abundancies • exotica: DCC, parity violation, …

  18. => Talk by D. d’Enterria The discovery of high pT suppression at RHIC

  19. What happens at higher transverse momenta? PHENIX and STAR extend measurements to ~ 10 GeV

  20. Centrality Dependence vs pT Phobos Phobos

  21. Is this the jet quenching in QGP? Very likely; .but could there be alternative explanations? (2002)lternative Bjorken; Gyulassy, Wang; Baier, Dokshitzer, Mueller, Peigne, Schiff; Wiedemann, Salgado; Vitev, Levai, … DK, Levin, McLerran hep-ph/0210332 Yes, possibly: 1) Small x evolution leads to .the modification of gluon propagators - “anomalous dimension”: 2) Qs is the only relevant dimensionful parameter in the CGC; .thus everything scales in the ratio Tthe A-dependence is changed => Npart scaling! 3) Since

  22. D-Au collisions: suppression or enhancement?

  23. RdAu vs pT Phobos Central Au+Au

  24. p+p vs. d+Au No “data manipulation” => Talk by F. Wang D. Hardtke, STAR Coll. • Azimuthal correlations are qualitatively consistent • Quantitative evaluation will constrain • Nuclear kT from initial state multiple scattering • Shadowing • Models that predict “monojets” due to initial state effects ruled out

  25. Conclusion:high pT suppression is a final-state effect Can one prove that it is due to a radiative jet energy loss In the Quark-Gluon Plasma? Quite likely: one possibility is to use the heavy quarks Yu.Dokshitzer, DK ‘01 => Talk by N. Armesto Radiation off heavy quarks is suppressed (“dead cone”) => less quenching On the other hand, D mesons have about the same size as .pions and kaons, and so in the hadron absorption scenario .the suppression should be the same

  26. However, the arguments for the CGC-caused,suppression should hold for sufficiently small x; Does this happen at RHIC? Study the forward rapidity region: Moving to y=+4 from y=0 increases the saturation scale .by factor of three RHIC results: Talks by B. Cole, E. Kistenev, M. Liu, F. Wang

  27. Expectations for RdAu at large rapidity Agreement on the presence of suppression due to the quantum Small x evolution in the CGC picture: DK, E. Levin and L. McLerran, hep-ph/0210332; R. Baier, A. Kovner, U. Wiedemann, hep-ph/0305265 v2 DK, Yu.Kovchegov and K. Tuchin, hep-ph/0307037 v2 J. Albacete, N. Armesto, A. Kovner, C. Salgado, U. Wiedemann, hep-ph/0307179; Agreement on the presence of Cronin effect in the classical ,approach and in the multiple scattering picture: L.McLerran and R.Venugopalan; Yu.Kovchegov and A.H.Mueller; J. Jalilian-Marian; A. Dumitru; F. Gelis;… X.N.Wang; M. Gyulassy; I. Vitev;…

  28. Model predictions D. Kharzeev, Yu. Kovchegov and K. Tuchin, hep-ph/0307037 I. Vitev nucl-th/0302002 v2 CGC at y=0 Y=0 As y grows Y=3 Y=-3 Very high energy R. Debbe, BRAHMS Coll., Talk at DNP Meeting, Tucson, November 2003

  29. d-Au Nuclear Modification factor at  ~3.2 RdAu compares the yield of negative particlesproduced in dAu to the scaled number of particles with same sign in p-p The scale is the number of binary collisions: Ncoll=7.2 (minimum biased) PRL 91 072305 (2003) BRAHMS preliminary R. Debbe, BRAHMS Collaboration, Talk at the DNP Meeting, Tucson, November 2003

  30. RdAu at different rapidities Number of binary collisions in minimum biased events is estimated: Ncoll = 7.2±0.3 Statistical errors dominant over the systematic ones at =2 and 3 Systematic error (not shown) ~15% The values for =0 were published in: PRL 91 072305 (2003) All ratios extracted from minimum biased data samples R.Debbe, BRAHMS, QM’04

  31. Centrality dependence All numerators and denominator are scaled by the appropriate estimated number of binary collisions (HIJING + BRAHMSGEANT) The ratios are corrected for trigger inefficiency. All other corrections (acceptance, tracking efficiency.. ) cancel out. R.Debbe, BRAHMS, QM’04

  32. d Au Centrality Dependence of Particle Production @Fwd/Bwd Directions • Stopped hadrons • Mesons + Baryons • Light mesons • Pions + Kaons • Heavy flavors • Charm + Beauty Ming Liu, PHENIX, QM’04 => Talk by M. Liu

  33. d+Au RCP at forward rapidities • Au-Side RCP shows almost no variation with centrality • d-side is interesting: more central is more suppressed RCP L.Barnby, STAR, QM’04 PT [GeV/c]

  34. d Au spectra at (not so) forward rapidity P. Steinberg, PHOBOS, QM’04

  35. Rapidity dependence of RdAu

  36. Is this “just shadowing”? Yes, it is - shadowing is the deviation of nuclear parton distributions from the nucleon’s ones. CGC is a theory of shadowing Conventional parameterizations of shadowing have so far failed in describing the rapidity and centrality dependence of the data (EKS, de Florian, Accardi-Gyulassy,… Some approaches (e.g. HIJING shadowing) describe suppression at y=3, but also predict it at y=0. NB:approaches relating shadowing to diffraction should work

  37. Further tests of quantum evolution in the CGC • Back-to-back correlations for jets separated by several units of rapidity are very sensitive to the evolution effects A.H.Mueller,H.Navelet and to the presence of CGC DK, E.Levin,L.McLerran, hep-ph/0403271 • Open charm, dileptons, photons DK, K.Tuchin, hep-ph/0310.. in the forward region R.Baier,A.H.Mueller,D.Schiff, hep-ph/0403201

  38. Phase diagram of high energy QCD

  39. Summary High energy heavy ion program allows to study QCD in the domains of high parton density (small x, large A) and high energy density Novel exciting effects have been observed in Au-Au and forward d-Au interactions; they point towards the collective partonic behavior and non-linear dynamics at small x

  40. Back-up slides

  41. PhenixPreliminary Au d Stopped Hadrons!

  42. PhenixPreliminary Au d Stopped Hadrons!

  43. PhenixPreliminary Cronin effect & anti-shadowing? d Shadowing? Au Stopped Hadrons!

  44. RCP(y): Muons from Light Meson Decays PhenixPreliminary RCP

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