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Status of Quark-Gluon Plasma and saturation effects at RHIC

Status of Quark-Gluon Plasma and saturation effects at RHIC. Introduction Status of QGP at RHIC  Particle multiplicities  Elliptic flow  High p t suppression & jet quenching High density gluon saturation (CGC)  d-Au data  Forward rapidities Summary & perspectives.

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Status of Quark-Gluon Plasma and saturation effects at RHIC

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  1. Status of Quark-Gluon Plasma and saturation effects at RHIC • Introduction • Status of QGP at RHIC Particle multiplicities Elliptic flow High pt suppression & jet quenching • High density gluon saturation (CGC) d-Au data Forward rapidities • Summary & perspectives Fouad RAMI Institut de Recherches Subatomiques, Strasbourg F.Rami, IReS Strasbourg Sinaia2005

  2. •Explore and characterize the QGP •Study QCD matter at high densities Main Goals in RHIC experiments Phase Diagram of Nuclear Matter F.Karsch, hep-lat/0106019 Lattice QCD Energy Density/T4  TC  160MeV (B = 0) Temperature  Large experimental program F.Rami, IReS Strasbourg Sinaia2005

  3. Two independent rings ~3.8 km in circumference SNN (GeV) L(cm-2 s-1) p+p 500 1032 Au+Au 200 1026 Relativistic Heavy Ion Collider @ BNL BRAHMS PHOBOS • First Physics Run: June 2000 PHENIX • 2000-2005: 5 runs STAR • Several systems/energies Au+Au @ 200 GeV @ 130 GeV @ 62.4 GeV Cu+Cu @ 200 GeV @ 63 GeV d+Au @ 200 GeV 62.4 GeV • p+p @ 200 GeV • (reference data) RHIC accelerates all species from p to Au F.Rami, IReS Strasbourg Sinaia2005

  4. Particle multiplicities at RHIC Central Au+Au event measured by STAR/TPC, @ 130 GeV  Very large number of charged particles per event dNch/d|=0~ 650 (at 200GeV)  much higher than at SPS Number of charged particles per unit of rapidity at =0 F.Rami, IReS Strasbourg Sinaia2005

  5. Wang & Gyulassy, PRL86(2001)3496   RHIC (average) BRAHMS   |  | | |  =0 dNch/d at Mid-RapidityEnergy Dependence • Large increase from SPS to RHIC (almost a factor of 2)  Higher energy densities • An estimate of ε ~ 5 GeV/fm3 at 200GeV (Bjorken model) PHENIX, PRL87(2001)052301  Well above the critical density (~ 1 GeV/fm3) • Au+Au data much larger than pp  Not a simple superposition Medium effects  important role in AA collisions F.Rami, IReS Strasbourg Sinaia2005

  6. Semi-central Collisions z y x Elliptic Flow • Elliptic Flow • Pressure converts spatial • anisotropy into p-space • anisotropy Reaction plane STAR, PRL90(2003)032301  MR Collective Flow Collective expansion of Nuclear Matter following the compression phase • Fourier decomposition of the azimuthal distributions dN/d = F0 (1 + 2vicos(i))  Response to early pressure v2 :2nd harmonic Fourier coefficient  Measure of Elliptic Flow F.Rami, IReS Strasbourg Sinaia2005

  7. STAR, PRL86(2001)402 • Large set of v2 data available • at RHIC (STAR, PHENIX) for • ≠ particle species Au+Au @ 130GeV Hydro limit • All can be reproduced by hydro • including the mass dependence • Hydro  good agreement for • soft particles: • pions up to pt~ 1.5GeV/c • protons up to pt~ 2.5GeV/c  More than 95% of the emitted particles Pb+Pb at SPS The bulk of the fireball behaves hydrodynamically • To reproduce the large v2 values  Hydro evolution must start very early  Fast thermalization Peripheral Central Elliptic Flow at RHIC • Much larger elliptic flow at RHIC •  high degree of thermalization • (multiple interactions of • produced particles) • Supported by the good agreement with hydrodynamical model F.Rami, IReS Strasbourg Sinaia2005

  8. Schematic view of jet production Yield(AA) RAA = hadrons leading particle q q • In A-A, partons traverse the medium leading particle • If QGP  partons will lose a large part of their energy (induced gluon radiation)  Suppression of jet production  Jet Quenching Nuclear Modification Factor NCOLL(AA)  Yield(pp) Scaled pp reference High pt Suppression & Jet Quenching • Particles with high pt’s (above ~2GeV/c) are primarly produced in hard scattering processes early in the collision  Probe of the dense and hot stage • p+p experiments  Hard scattered partons fragment into jets of hadrons Experimentally  Suppression in the high pt region of hadron spectra (relative to p+p) F.Rami, IReS Strasbourg Sinaia2005

  9. PHENIX, PRL88(2002)022301 (h++h-)/2 0 RHIC High pt Suppression at RHIC • At RHIC  Significant suppression New phenomenon at RHIC • Not observed at lower energies SPS(Pb+Pb)  Enhancement  due to initial state multiple scattering (Cronin effect) well known in p+A collisions • Suppression  consistent with partonic energy loss (Quenching) But, it might be also due to saturation of gluon densities (initial state effect)  Jets do not lose energy but they are produced in a smaller number Compare A+A and d+A (Run3, Control experiment) • Gluon sat.  Suppression in dAu • Quenching  No suppression in dAu F.Rami, IReS Strasbourg Sinaia2005

  10. Initial or Final State effect ? Same conclusion Final State effects are dominant in central Au+Au at RHIC as expected from the formation of a hot and dense medium of partonic matter F.Rami, IReS Strasbourg Sinaia2005

  11. Large particle multiplicities • High energy densities (well above critical) • High degree of collectivity and early thermalization • Presence of a dense partonic medium Summary of the main experimental observationsfor central Au+Au collisions  All of these results are consistent with the existence of a dense partonic state of matter characterized by strong collective interactions Main conclusion of the 4 RHIC White Papers (to be published in Nucl.Phys.A) F.Rami, IReS Strasbourg Sinaia2005

  12. Gluon Density x =0 • No apparent sign of saturation in • high pt hadron spectra for d+Au Those data are for MR particles  More forward rapidities (smaller x values) High Density Gluon Saturation at RHIC • Several global features of Au+Au and d+Au • collisions at RHIC can be reproduced by the • Color Glass Condensate model (high density • gluon saturation in the initial state) McLerran, hep-ph/0402137 As x becomes smaller and smaller, the gluon density increases faster  driving force toward saturation BRAHMS F.Rami, IReS Strasbourg Sinaia2005

  13. Forward measurements in d+Au collisions Sensitivity to smaller-x values MRS d Au FS • To reach small x in the gluon distribution of the Au nucleus xAu = mt/S e-y  Go very forward • BRAHMS spectrometers measure in the d-fragmentation region From y=0 to y=4  x values lower by ~10-2  One could hope to see the occurrence of a suppression effect D.Kharzeev et al, hep-ph/0307037 F.Rami, IReS Strasbourg Sinaia2005

  14. What do we expect? D. Kharzeev et al, hep-ph/0307037 CGC at y=0 As y grows Very high energy RpA : Nuclear Modification Factor • At RHIC energies Cronin effects predominant at mid-rapidity • At more forward y’s •  Transition from • Cronin enhancement • to a suppression • effect • This is what one would expect if there is an effect of gluon density saturation in the initial state F.Rami, IReS Strasbourg Sinaia2005

  15. BRAHMS, PRL 93 (2004) 242303 • η=0, (h++h-)/2 η=3.2, h- x ~ 10-2 What do we see in the data? For pt=2 GeV/c (θ=4deg) x ~ 510-4 • Transition from Cronin enhancement to suppression • Qualitatively consistent with the expected behavior for CGC F.Rami, IReS Strasbourg Sinaia2005

  16. BRAHMS, PRL 93 (2004) 242303 • η=0, (h++h-)/2 • η=1,(h++h-)/2 η=2.2, h- η=3.2, h- x ~ 10-2 (for pt=2GeV/c) (Min bias) • Suppression increases with rapidity as expected for saturation effects Rapidity Dependence F.Rami, IReS Strasbourg Sinaia2005

  17. Yield(0-20%)/NCOLL(0-20%) RCP = Yield(60-80%)/NCOLL(60-80%) Au-side d-side Central [0-20%]  BRAHMS  PHENIX (hadrons) nucl-ex/0411054 Results from other RHIC experiments Reference from peripheral collisions • Good agreement between BRAHMS and PHENIX • PHOBOS  consistent results (limited y-range) F.Rami, IReS Strasbourg Sinaia2005

  18. Comparison to CGC calculations D. Kharzeev at al. hep-ph/0405045 =0 =1 • Overall good agreement • Calculations predict also a transition from Cronin enhancement at MR to suppression at larger y’s • So far  No alternative explanations within realistic model calculations =2.2 =3.2 • Quantitative CGC calculations for d+Au @ SNN=200 GeV Nuclear Modification Factor Nuclear Modification Factor F.Rami, IReS Strasbourg Sinaia2005

  19. Summary & Perspectives • Results obtained so far at RHIC for central Au+Au collisions are consistent with the formation of a dense partonic state of matter characterized by strong collective interactions • Strong hints of saturation effects at RHIC (from d+Au data)  CGC might provide the initial conditions for A-A collisions at RHIC The task now  • Characterize the properties of this dense partonic state of matter • Confirmation of the Color Glass Condensate  will require further experimental tests (more sensitive probes) • RHIC (upgrades  improved physics capabilities) • LHC  much higher energies (smaller x) F.Rami, IReS Strasbourg Sinaia2005

  20. Backup slides F.Rami, IReS Strasbourg Sinaia2005

  21. Future experimental progran at RHIC • Next 5 years  Significant detector upgrades • Improved vertexing for charm measurements • Better particle id (TOF) • Low-mass dilepton measurements • Expanded forward coverage ( low-x physics) • Longer term  Significant upgrade of the machine (RHIC II) based on electron cooling ( higher luminosities) • Additional upgrades • Proposal for a new detector to exploit the increased luminosity  Extend jet-related measurements to much higher pt’s (into the perturbative regime) Physics program of RHIC II still under discussion F.Rami, IReS Strasbourg Sinaia2005

  22. Main Goals in RHIC experiments Phase Diagram of Nuclear Matter F.Karsch, hep-lat/0106019 Lattice QCD Energy Density/T4  TC  160MeV (B = 0) Temperature B = F/NB •Explore and characterize the QGP F = Free energy NB = Baryonic Number (baryon – anti-baryon) •Study QCD matter at high densities  Large experimental program F.Rami, IReS Strasbourg Sinaia2005

  23. Space-time evolution of a heavy-ion collision at collider energies • There are several stages in the collision Emission of hadrons (t  20fm/c) Parton interactions take place during first stages Initial State (v~c) Dense Medium CGC ? F.Rami, IReS Strasbourg Sinaia2005

  24. Two “Large” Detectors at RHIC STAR Solenoidal field Large Solid Angle TPC Si-Vertex detector RICH, EM Cal, TOF PHENIX Axial Field High Resolution & Rates 2 Central Arms, 2 Forward Arms TEC, RICH, EM Cal, Si, TOF, -ID • Measurements of hadronic observables with a large acceptance and Pid • Event-by-event analyses • Designed to measure simultaneously Leptons, Photons, and Hadrons in selected solid angles • Rare signals such as J/ψ decaying into muons and electrons, direct photons F.Rami, IReS Strasbourg Sinaia2005

  25. Two “Small” Detectors at RHIC PHOBOS Nearly 4 coverage with Si-detectors 2 Arm Spectrometers (also based on Si) Paddle Trigger Counter TOF Spectrometer Octagon+Vertex Ring Counters • Total charged particle multiplicity in 4 • & Global properties (elliptic flow) • Charged hadron spectra (small acceptance) BRAHMS 2 spectrometers (movable) Magnets, Tracking Chambers, TOF, RICH • Detailed measurements of momentum spectra and yields of charged hadrons over a wide range of rapidities (including the forward kinematical region) F.Rami, IReS Strasbourg Sinaia2005

  26. Thermal model parameters from particle ratios • Statistical Model Analysis • Analysis of particle ratios measured at RHIC in a grand canonical ensemble with baryon number, strangeness and charge conservation P.Braun-Munzinger et al, PLB518(2001)41 Thermal model parameters at chemical freeze-out B = 46±5 MeV T=174±7 MeV • Similar analysis at SNN=200GeV B = 29±8 MeV T=177±7 MeV SNN=130GeV Agreement -> indicates a high degree of chemical equilibration Flow -> hydro (thermalisation ..) F.Rami, IReS Strasbourg Sinaia2005

  27. Wang & Gyulassy, PRL86(2001)3496 1  3  RHIC (average) BRAHMS 2   |  | | |  =0 1 HIJING – Jet quenching 2 HIJING – No Jet quenching 3 EKRT (Gluon Saturation) dNch/d at Mid-RapidityEnergy Dependence • Saturation models also reproduce the measured multiplicities F.Rami, IReS Strasbourg Sinaia2005

  28. BRAHMS, PLB523(2001)227 BRAHMS, PRL88(2002)202301 0-5% 30-40% SNN=130GeV SNN=200GeV Nch(-4.7<<4.7) 3860  300 4630  370 Very high charged hadron multiplicities dNch/d|=0 = 553  36 dNch/d|=0 = 625  55 Number of charged particles per unit of rapidity in the MR (at =0) • Much higher multiplicities than at CERN-SPS (Pb+Pb) F.Rami, IReS Strasbourg Sinaia2005

  29. PHENIX, PRL87(2001)052301 Au+Au @ SNN=130GeV MB Central (top 5%) Very high energy densities • Transverse energy distributions measured by PHENIX (calorimetry) • Central events  dET/d|=0~ 500 GeV (  SPS) • Using Bjorken estimate for the energy density (J.D.Bjorken, PRD27(83)140)  BJ~ 4.6 GeV/fm3 • At 200GeV)  BJ ~ 5 GeV/fm3 factor of ~1.6 larger than at SPS BJ ~ 3.2 GeV/fm3 for Pb+Pb at SPS (NA49, PRL75(1995)3814) Well above the value expected for the Critical Energy Density (crit ~ 1 GeV/fm3) F.Rami, IReS Strasbourg Sinaia2005

  30. pR2 Energy Density from Transverse Energy Measurements Bjorken formula for thermalized energy density J.D.Bjorken PRD27(83)140 time to thermalize the system (t0 ~ 1 fm/c) ~6.5 fm Longitudinal expansion of thermalized system F.Rami, IReS Strasbourg Sinaia2005

  31.    EVENT CHARACTERIZATION COLLISION CENTRALITY Au+Au @ SNN=130GeV • Measured with Multiplicity Detectors (TMA and SiMA) Central b=0 Peripheral b large Central Peripheral • Define Event Centrality Classes  Slices corresponding to different fractions of the cross section • For each Centrality Cut  Evaluate the corresponding number of participants Npart and number of inelastic NN collisions NCOLL (from Glauber Model) F.Rami, IReS Strasbourg Sinaia2005

  32. Elliptic Flow: pt dependence Kolb & Heinz, nucl-th/035084  Good agreement for central and mid-central events But overpredicts v2 for peripheral events (b  10fm)  incomplete thermalization F.Rami, IReS Strasbourg Sinaia2005

  33. Elliptic Flow from Parton Cascade Transport Calculations Molnar & Gyulassy, NPA 697 (2002) 495 • Calculations with ≠ transport opacities ζ • Agreement with data if ζ is very large •  large number of interactions • among the fireball constituents • (partons) • ζ very large  hydro limit (pt < 1.5GeV/c) • Parton cascade predicts • saturation at high pt’s • (observed in the data) • High pt particles escape the • fireball before having suffered • a sufficient number of rescattering • to thermalize their momenta. F.Rami, IReS Strasbourg Sinaia2005

  34. Parton Cascade Model (AMPT,Zhang et al, PLB455(1999)45) v2 Au+Au at 200GeV time(fm/c) Flow is Sensitive to Early Stages  Elliptic flow builds up in the first instants of the collision (before hadronization) and then stays constant • Rescattering converts the initial space • anisotropy of the overlap region to the • momentum anisotropy of elliptic flow •  v2 is sensitive to the number of • interactions and can be considered • as a measure of thedegree of • thermalization at early time. v2 is proportional to the parton-parton scattering cross section used in the calculations F.Rami, IReS Strasbourg Sinaia2005

  35. Elliptic Flow: Sensititivity to the EoS Hydro calculations: Huovinen, Kolb & Heinz NPA698 (2002) 475 • Elliptic flow builds up and saturates • early in the collision •  sensitivity to high density EOS  Hydrodynamical mass splitiing (observed in the data) underpredicted by EOS/H EOS/Q  quark gluon plasma EOS (hard) EOS/H  pure hadron resonance gas (soft) F.Rami, IReS Strasbourg Sinaia2005

  36. Yield(AA) RAA = Nuclear Modification Factor Scaled pp reference RAA<1  Suppression relative to scaled pp reference NCOLL(AA)  Yield(pp) Nuclear Modification Factor RAA PHENIX, PRL91(2003)072301 Scaled pp reference F.Rami, IReS Strasbourg Sinaia2005

  37. Nuclear Modification Factor RAA For peripheral collisions  NCOLL scaling works well Nucl-ex/0410003 (PHENIX White paper)  NCOLLscaling of hard processes has been also checked using direct photons, which are produced via hard scattering processes but do not loose energy in the medium since they have no color charge F.Rami, IReS Strasbourg Sinaia2005

  38. Evaluation of Npart and NCOLL Npart: Nucleons that interact inelastically in the overlap region between the two interacting nuclei NCOLL : Number of binary nucleon-nucleon collisions (one nucleon can interact successively with several nucleons if they are in its path) • Use Glauber Model Nucl.Phys.B21(1970)135 Main assumption : Independent collisions of part. nucleons Nucleons suffer several collisions along their incident trajectory (straight-line) without deflection and without energy loss • Nucleons inside nuclei distributed according to a Woods-Saxon density profile • Interaction probability between 2 nucleons is given by the pp cross section • Calculate the overlap integral at a given impact parameter p+p : Np=1 and NCOLL=1 p+A : Np=1 and NCOLL>1 F.Rami, IReS Strasbourg Sinaia2005

  39. High pt suppression: Theoretical calculations Vitev & Gyulassy, hep-ph/0209161 • pQCD based calculations incorporating parton energy loss via medium induced gluon radiation are able to reproduce the data Energy dependence can be explained by the competition between quenching, nuclear shadowing and Cronin effect • Realistic hadronic calculations (Cassing, Gallmesiter and Greiner, hep-ph/0311358) •  unable to reproduce the observed effect F.Rami, IReS Strasbourg Sinaia2005

  40. d+Au Another evidence in favor of Jet Quenching Azimuthal Correlations STAR, PRL91(2003)072304 • Azimuthal correlations between • a high pt particle (trigger) with • 4  pt  6 GeV/c and all other • particles with pt above 2 GeV/c •  Indirect way to identify the • formation of jets • p+p  Clear two-jet signal • (back to back correlation) • d+Au  The signal survives • Central Au+Au  The signal disappears • Strong experimental evidence for Jet Quenching in Au+Au Jets are deflected in the medium  destroys the coplanarity of the 2 jets F.Rami, IReS Strasbourg Sinaia2005

  41. Low energy Gluon Density Large x Gluon density increases x High energy Small x x = Econstituant/Ehadron High Density Gluon Saturation • e-p scattering at HERA Gluon density in a proton increases strongly from large x to small x (x=fraction of E transfered to the gluon) Saturation at high density QS : Saturation scale F.Rami, IReS Strasbourg Sinaia2005

  42. McLerran, hep-ph/0402137 F.Rami, IReS Strasbourg Sinaia2005

  43. Forward measurements in d+Au collisions Sensitivity to smaller-x values MRS d Au FS • BRAHMS spectrometers measure in the d-fragmentation region • To reach small x in the gluon distribution of the Au nucleus xAu = mt/S e-y  Go very forward  Larger saturation scaleQS : Qs2(x) = Q02 (x0/x)λ • Qs2  A1/3(Thickeness effect)  Saturation scale in Au larger than in p (saturation can be probed at lower x) • No final state effects in d+Au From y=0 to y=4  x values lower by ~10-2  One could hope to see the occurrence of a suppression effect D.Kharzeev et al, hep-ph/0307037 F.Rami, IReS Strasbourg Sinaia2005

  44. Kinematics of p-Au xB xA rapidity A is p and B is Au Energy and momentum conservation xL = xA - xB =(2MT/√s)sinh y kA + kB = k xAxB = MT2/s A solution to this system is: xA = (MT/√s) ey xB = (MT/√s) e-y y is the rapidity of the detected particle (xL,k) xL is its logitudinal momentum fraction xA (xB) is the longitudinal momentum fraction of the projectile (target) parton F.Rami, IReS Strasbourg Sinaia2005

  45. Yield(0-20%)/NCOLL(0-20%) =3.2 RCP = Yield(60-80%)/NCOLL(60-80%) • More suppression in central events  Also consistent with CGC matter Centrality Dependence Reference from peripheral collisions Highest density of gluons in central collisions  Largest suppression F.Rami, IReS Strasbourg Sinaia2005

  46. CGC calculations: Predictions for LHC LHC, =0 RHIC, =3.2 Predictions for LHC p-A collisions • Stronger suppression • at LHC (smaller x) F.Rami, IReS Strasbourg Sinaia2005

  47. Accessible x range at RHIC and LHC X2 = [MQQ/SNN] exp(- yQQ) X1 = [MQQ/SNN] exp(+ yQQ) - - - - - - MQQ : Invariant mass of the QQ pair produced in the hard scattering yQQ : Rapidity of the pair - A.Dainese, nucl-ex/0311004 • LHC  higher energies, higher rapidities (smaller x) • p-A (and A+A) deeply inside the saturation regime • Possibility to probe saturation also in p+p F.Rami, IReS Strasbourg Sinaia2005

  48. Nuclear modification factor for h- and h+ F.Rami, IReS Strasbourg Sinaia2005

  49. Nuclear modification factor for mesons and baryons anti-proton data not corrected for anti-lambda feed down - Difference between baryons and mesons - Related to parton recombination? (Hwa et al, PRC71(2005)024902 F.Rami, IReS Strasbourg Sinaia2005

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