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Relativistic Heavy Ion Physics: An Experimental Review: 

Relativistic Heavy Ion Physics: An Experimental Review Saskia Mioduszewski 22 July 2003


Outline Physics Goals: deconfinement and chiral symmetry restoration Overview of the Program Global Observables charged-particle multiplicity flow Other Experimental Highlights J/y suppression low mass dilepton enhancement high pT suppression Summary

Lattice QCD at Finite Temperature: 

Lattice QCD at Finite Temperature Coincident transitions: deconfinement and chiral symmetry restoration F. Karsch, hep-ph/010314 Critical energy density: TC ~ 175 MeV  eC ~ 0.7 GeV/fm3 Ideal gas (Stefan-Boltzmann limit) (mB=0) Chiral symmetry spontaneously broken in nature. Quark condensate is non-zero: At high temperature and/or baryon density Constituent mass  current mass Chiral Symmetry (approximately) restored.

Schematic Phase Diagram of Strongly Interacting Matter: 

Schematic Phase Diagram of Strongly Interacting Matter Test QCD under extreme conditions and in large scale systems Search for deconfined QGP phase SISAGS  SPS RHICLHC From high baryon density regime to high temperature regime

How to Observe QGP in Heavy Ion Collisions: 

How to Observe QGP in Heavy Ion Collisions Some tools to distinguish QGP from dense hadron gas: Direct observation of deconfinement: suppression of J/ High energy density: interaction of jets with medium High temperature: direct photons/dileptons Chiral symmetry restoration: meson properties (m,) expected to be modified in medium Equilibration at early stage large pressure collective expansion: flow

History of High-Energy A+B Beams: 

History of High-Energy A+B Beams BNL-AGS: mid 80’s, early 90’s O+A, Si+A 15 AGeV/c sNN ~ 6 GeV Au+A 11 AGeV/c sNN ~ 5 GeV CERN-SPS: mid 80’s, 90’s O+A, S+A 200 AGeV/c sNN ~ 20 GeV Pb+A 160 AGeV/c sNN ~ 17 GeV BNL-RHIC: early 00’s Au+Au sNN ~ 130 GeV Au+Au, p+p, d+Au sNN ~ 200 GeV

The RHIC Experiments: 

The RHIC Experiments

Global Observables: 

Global Observables Reflect the conditions of the system after freeze-out, after resonance decays Charged-Particle Multiplicity helps constrain models reflects produced entropy Flow collective expansion, rescattering pressure

AA collisions are not all the same : 

AA collisions are not all the same Nuclei are extended objects Impact parameter Number of participants Centrality ( % from total inelastic cross-section)

Charged-Particle Rapidity Distribution: 

Charged-Particle Rapidity Distribution Enhancement of particle production for central collisions at mid-rapidity. Particle production scales with Npart at high rapidities (h >3). RHIC SPS BRAHMS


 From SPS to RHIC : * dNch/dy increases by ~70% at ÖsNN = 130 GeV * dNch/dy increases by ~90% at ÖsNN = 200 GeV ln(ÖsNN ) dependence from AGS to RHIC ÖsNN Dependence of dNch/dy

Radial Flow: 

Radial Flow – Expansion of system due to pressure – Heavier particles shifted to higher pT – Observable: <bT> from slopes of mT spectra as a function of mass – Spectra can be described by hydrodynamic models for pT< 2-3 GeV/c and mid-peripheral to central events

Single Particle Spectra (low pT): 

Single Particle Spectra (low pT) Decreasing slope for increasing particle mass and centrality T. Ullrich QM2002

Single Particle Spectra for most central events (0-5%): 

Single Particle Spectra for most central events (0-5%) proton yield ~ pion yield @ 2 GeV consistent with hydrodynamic model calculations (e.g. comparison to 130 GeV data - Teaney, Lauret, Shuryak nucl-th/0110037) PHENIX Preliminary PHENIX Preliminary Au+Au at sqrt(sNN) =200GeV Au+Au at sqrt(sNN) =200GeV J. Burward-Hoy, QM2002

Mean Transverse Momentum vs. Npart: 

Mean Transverse Momentum vs. Npart <pT> increases with Npart and particle mass, indicative of radial expansion Relative increase with Npart greater for (anti)p than for , K J. Burward-Hoy, QM2002 closed symbols: 200 GeV open symbols: 130 GeV

Hydrodynamic Model Fit to the Spectra: 

Hydrodynamic Model Fit to the Spectra PHENIX: Freeze-out Temperature Tfo = 110  23 MeV Transverse flow velocity bT = 0.7  0.2  < bT> ~ 0.5 Most central collisions for 200 GeV data Ref: E. Schnedermann, J. Sollfrank, and U. Heinz, Phys. Rev. C 48, 2462 (1993) Au+Au at sqrt(sNN) =200GeV STAR: Tfo ~ 100 MeV bT ~ 0.6 J. Burward-Hoy, QM2002

Mid-Rapidity mT spectra at SPS: 

Mid-Rapidity mT spectra at SPS M. van Leeuwen QM2002 (NA49) NA57, H. Helstrup, this conference: Tfo = 131 ± 10 MeV <bT> = 0.47 ± 0.02

Elliptic Flow in Non-central Collisions: 

Elliptic Flow in Non-central Collisions Early state manifestation of collective behavior: Asymmetry generated early in collision, quenched by expansion  observed asymmetry emphasizes early time Second Fourier coefficient v2:

Elliptic Flow: 

Strong elliptic flow signal   strong (collective) pressure Large and fast rescattering (early thermalization) v2 dependent on mass (predicted by hydro P. Huovinen et al, PLB 503 (2001) 58). Elliptic Flow

Elliptic Flow: 

Elliptic Flow SPS: v2 ~ 0.03 RHIC: v2 ~ 0.055 Wetzler QM2002 E877: Phys.Lett.B474:27-32, 2000 CERES: QM2001 INPC 2001 nucl-ex/0109017 STAR: PRC66 (2002) 034904 NA49 Preliminary 130 GeV data

Flow: Comparison of SPS and RHIC: 

Flow: Comparison of SPS and RHIC Radial Flow: pressure can build up over entire dynamics <bT> ~ 0.4 - 0.5 at SPS <bT> ~ 0.5 - 0.6 at RHIC Elliptic Flow: pressure must build up before asymmetry of system has diminished v2 ~ 0.03 at SPS v2 ~ 0.06 at RHIC Moderate increase in <bT>  more pressure at RHIC Significantly larger v2 is evidence for early build-up of pressure According to hydrodynamic models  early thermalization at RHIC (t~0.6fm/c - Heinz, Kolb Nucl.Phys.A702:269-280,2002 )

Energy Density: 

Energy Density Energy density a la Bjorken: dET/dy ~ 720 GeV (S. Bazilevsky QM2002, PHENIX PRELIMINARY) Estimate e for RHIC:

Other Highlights of Program: 

Other Highlights of Program Global observables  properties of collision dynamics, EOS Other probes for signatures of QGP J/y suppression  deconfinement low mass dileptons  chiral symmetry restoration high pT suppression  density of produced medium and energy loss

J/y suppression: probe of deconfinement: 

J/y suppression: probe of deconfinement • An “old” signature of QGP formation: (Matsui and Satz PL B178, (1986) 416). At high enough color density, the screening radius < binding radius  J/ will dissolve • Observation: Anomalous suppression in Pb-Pb collisions * beyond normal nuclear absorption abs ~ 4-6 mb

J/y suppression: Evidence of deconfinement?: 

J/y suppression: Evidence of deconfinement? L. Ramello, QM 2002 NA50 Preliminary Suppression increasing with centrality (discontinuities?) Exceeds normal nuclear absorption (as measured in p+A) Many models exist (hadronic and QGP) – data consistent with suggested QGP signature (Matsui, Satz, Kharzeev)

Charmonium (J/Y) physics at RHIC: 

possible signature of the deconfinement phase transition J/Y yield can be suppressed more than at SPS - dissolve in QGP (longer lifetime, higher temperature than SPS) enhanced - cc coalescence as the medium cools (2 orders of magnitude more production of cc pairs at RHIC) important to measure J/Y in p+p and d+Au to separate “normal” nuclear effects shadowing nuclear absorption in cold matter J/Y measurements in leptonic decay channels J/Y  e+ e- and J/Y  m+ m- in p+p at s = 200 GeV J/Y  e+ e- in Au+Au at sNN = 200 GeV Charmonium (J/Y) physics at RHIC (hep-ex/0307019) (nucl-ex/0305030)

J/Y Production at RHIC: 

J/Y Production at RHIC PHENIX, sNN = 200 GeV J/Y-Suppression maybe most compelling QGP evidence at CERN SPS Expectation at RHIC energies unclear 10 cc pairs produced per central Au+Au collision Possibly enhanced J/Y- production due to charm-coalescence - PLB477(2000) 28 normalized to PHENIX p+p measurement

Model comparisons : 

Model comparisons models that predict enhancement relative to binary collision scaling are disfavored no discrimination between models that lead to suppression

Low-Mass e+e- pairs: 

Low-Mass e+e- pairs Main CERES Result: Strong enhancement of low-mass pairs in A-A collisions (wrt to expected yield from known sources) Enhancement factor (.25 <m<.7GeV/c2): 2.6 ± 0.5 (stat) ± 0.6 (syst)


Interpretations  annihilation: +-  *  e+e- (thermal radiation from HG) Cross section dominated by pole at the  mass of the  em form factor: Plus or Add

Onset of Chiral Symmetry Restoration?: 

Onset of Chiral Symmetry Restoration? Dropping -meson mass (Rapp, Wambach et al) In-medium -meson broadening (G.E. Brown et al) What happens as chiral symmetry is restored? Dropping mass or broadening (melting)?

Fate of Hard Scattered Partons in Au+Au Collisions: 

Fate of Hard Scattered Partons in Au+Au Collisions Hard scatterings in nucleon-nucleon collisions produce jets of particles. In the presence of a color-deconfined medium, the partons strongly interact (~GeV/fm) losing much of their energy. “Jet Quenching”

Nuclear Modification Factor RAA : 

Nuclear Modification Factor RAA in absence of nuclear effects RAA < 1 at low pT (soft physics regime) RAA = 1 at high pT (hard scattering regime) “suppression” (enhancement, e.g. Cronin effect) RAA < 1 (> 1) at high pT Nuclear Modification Factor <Nbinary>/sinelp+p NN cross section

RAA for p0: 

By definition, processes that scale with Nbinary will produce RAA=1. RAA is what we measure divided by what we expect. RAA is < 1 at RHIC, but > 1 at SPS SPS: “Cronin” effect dominates RHIC: suppression dominates RAA for p0 Nbinary-scaling A.L.S.Angelis PLB 185, 213 (1987) WA98, EPJ C 23, 225 (2002) PHENIX, PRL 88 022301 (2002) PHENIX submitted to PRL, nucl-ex/0304022

Jet Quenching ?: 

Jet Quenching ? high pT suppression reproduced by models with parton energy loss other explanations not ruled out, need to measure initial-state effects Au+Aup0+X at sNN = 200 GeV Wang: X.N. Wang, Phys. Rev. C61, 064910 (2000). Levai: P.Levai, Nuclear Physics A698 (2002) 631. Vitev: I. Vitev and M. Gyulassy, hep-ph/0208108 + Gyulassy, Levai, Vitev, Nucl. Phys. B 594, p. 371 (2001).

RAA for p0 and charged hadrons: 

RAA for p0 and charged hadrons PHENIX AuAu 200 GeV p0 data: nucl-ex/0304022, submitted to PRL. charged hadron (preliminary) : NPA715, 769c (2003).

Azimuthal distributions in Au+Au: 

Azimuthal distributions in Au+Au Near-side: peripheral and central Au+Au similar to p+p Strong suppression of back-to-back correlations in central Au+Au collisions

RAA vs. RdA for charged hadrons and p0: 

RAA vs. RdA for charged hadrons and p0 No Suppression in d+Au, instead small enhancement observed (Cronin effect)!! d-Au results rule out initial-state effects as the explanation for Suppression at Central Rapidity and high pT Initial State Effects Only Initial + Final State Effects PHENIX (d+Au) hep-ex/0306021 submitted to PRL

Azimuthal distributions: 

Azimuthal distributions Near-side: p+p, d+Au, Au+Au similar Back-to-back: Au+Au strongly suppressed relative to p+p and d+Au Suppression of the back-to-back correlation in central Au+Au is a final-state effect

High pT Measurements at RHIC: 

High pT Measurements at RHIC d+Au collisions: No suppression at high pT Away-side jet strength consistent with p+p collisions Peripheral Au+Au collisions: Hadron yields consistent with Nbinary-scaled yields in p+p collisions Away-side jet strength consistent with p+p collisions Central Au+Au collisions: Hadrons are suppressed at high pT (up to 10 GeV/c) Away-side jet disappears Particle Composition in Central Au+Au collisions: What is happening with the protons?

Particle Species Dependence of High pT Suppression: 

Particle Species Dependence of High pT Suppression No apparent proton suppression for 2-4 GeV/c different production mechanism ? (Similar effect seen in STAR for  vs. Kshort suppression)

Particle Composition at High pT: 

Particle Composition at High pT p/p < 0.25 expected from jet fragmentation observed p/p ~ 0.4 in peripheral, ~ 1 in central protons from non-fragmentation sources ? nucl-ex/0305036


Summary Physics highlights: Strong collective expansion at SPS and RHIC Evidence for early equilibration at RHIC SPS: * Anomalous J/ suppression * Enhancement of low-mass dileptons RHIC: * Suppression of high pT particles and disappearance of away-side jet Very intriguing results. All consistent with QGP formation

Extra Slides: 

Extra Slides

Direct Photons (I): 

Direct Photons (I) Evidence for direct photons in central Pb-Pb collisions? 10-20% excess but 1 effect only CERES preliminary result: enhancement = 12.4% ± 0.8% (stat) ± 13.5% (syst) WA98

Direct Photons (II): 

Direct Photons (II) Comparison to scaled pA: similar spectrum but factor of ~2 enhanced yield in Pb-Pb, again ~1 effect. pQCD underpredicts direct photon yield WA98

Direct Photons: 

Direct Photons Direct Photons: Photons not originating from hadron decays like p0gg gall = gdirect + gdecay Direct photon signal seen in Pb+Pb at sNN=17.3 GeV Stronger signal expected at RHIC, because p0 suppressed by factor 5 Suppression appears to be a final state effect Direct photons not affected by final state interactions pQCD calculation for direct g and p0 in p+p at s=200 GeV (Werner Vogelsang):

Direct Photon Search: 

Direct Photon Search Au+Au at sNN = 200 GeV No direct photon signal seen within errors With further analysis systematic errors will be reduced ...

Azimuthal asymmtery (v2) at high pT: 

Azimuthal asymmtery (v2) at high pT Finite v2 up pT ~ 10 GeV Hydrodynamics up to pT ~ 2-3 GeV Jets correlated to reaction plane?

Neutral Pion Production in central and peripheral Au+Au collisions: 

Neutral Pion Production in central and peripheral Au+Au collisions reference p+p data with same detector binary scaling in peripheral Au+Au suppression factor ~ 5 in central Au+Au Binary scaling Participant scaling ×1/5 p0 at sNN = 200 GeV nucl-ex/0304022, submitted to PRL

Particle Spectra Evolution: 

Particle Spectra Evolution

Centrality Dependence: 

Centrality Dependence Dramatically different and opposite centrality evolution of Au+Au experiment from d+Au control. High pT hadron suppression in AuAu is due to a final state effect. Au + Au Experiment d + Au Experiment

What might all this mean?: 

What might all this mean? Conjecture: core of reaction volume is opaque to jets  surface emission Consequences: near-side fragmentation independent of system suppression of back-to-back jets suppression of inclusive rates strong elliptic flow at high pT Compelling picture, but is it right?

J/y suppression: Evidence of deconfinement?: 

J/y suppression: Evidence of deconfinement? PLB 477 (2000) 28 NA50 preliminary L. Ramello, QM 2002 melting of charmonium states: c (binding energy  250 MeV) and J/ (650 MeV)

Jet correlations: Au+Au vs. p+p: 

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


Centrality Determination For example, in PHENIX: Use combination of Zero Degree Calorimeters Beam-Beam Counters (sensitive to 92% of geom) to define centrality classes Glauber modeling to extract N-participants PHENIX

Centrality Dependence: Comparison to Models: 

Centrality Dependence: Comparison to Models Saturation models reproduce the scaling with centrality and energy dependence! dNch/dh/(0.5Npart) Kharzeev & Levin, nucl-th/0108006 Schaffner-Bielich et al, nucl-th/0108048

- Centrality Dependence of Pion Suppression -: 

- Centrality Dependence of Pion Suppression - smooth increase of suppression with centrality neither binary or participant scaling p0 at sNN = 200 GeV nucl-ex/0304022, submitted to PRL

The SPS Experiments: 

The SPS Experiments 1986 - 1987 : Oxygen @ 60 & 200 GeV/nucleon 1987 - 1992 : Sulphur @ 200 GeV/nucleon 1994 - 2000 : Lead @ 40, 80 & 158 GeV/nucleon 2002 - 2003 : Indium and Lead @ 158 GeV/nucleon And proton beams for pp and pA reference studies Carlos Lourenco QM01

Color Glass Condensate: 

Color Glass Condensate Alternate Explanation Nucleons contain many low x partons. At some scale, and particular to relativistically contracted nuclei, gluons will saturate phase space and essentially cancel. Jets are not quenched, but are apriority made in fewer numbers. Color Glass Condensate hep-ph/0210033 Gribov, Levin, Ryshkin, Mueller, Qiu, Kharzeev, McLerran, Venugopalan, Balitsky, Kovchegov, Kovner, Iancu High x Low x

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