1 09 ISMD31

Overview of Relativistic Heavy-Ion Physics: 

Overview of Relativistic Heavy-Ion Physics ISMD31 Datong, China, Sept. 1-7, 2001 Itzhak Tserruya Weizmann Institute

Outline: 

Outline • Introduction * Main Goals: Deconfinement and Chiral Symmetry Restoration * Experiments and Program • Global and Hadronic Observables * From AGS  SPS  RHIC • Summary • Highlights of the Programme * J/ Suppression * Low-Mass Dileptons and Photons * High pt

Two Fundamental Issues:Deconfinement and Chiral Symmetry Restoration: 

Two Fundamental Issues: Deconfinement and Chiral Symmetry Restoration QCD Color charges: V(r)  r Long range  confinement color insulator HIGH DENSITY

Deconfinement Phase Transition: 

Deconfinement Phase Transition 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 (neutron stars) to high temperature regime (early universe)

Chiral Symmetry Restoration: 

Chiral Symmetry Restoration 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. Meson properties (m,) expected to be modified  meson best candidate due to small lifetime  = 1.3fm/c

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

The RHIC Experiments: 

The RHIC Experiments

Physics Menu: 

Physics Menu * Needs upgrade

How is RHIC Different?: 

How is RHIC Different? • Collider * No target, no delta electrons ! • Dedicated machine * Will run 20-30 weeks per year • High Energy * Access to perturbative phenomena (Jets) • Comprehensive detectors * Measure same observables in several detectors with significant overlap for comparisons. • Very short history * 2000 Au -- Au sNN = 130 GeV/c * 2001 Au -- Au sNN = 200 GeV/c

Spectacular Events: 

Spectacular Events CERES TPC @ SPS STAR TPC @ RHIC Pb – Au sNN = 17 GeV/c Au – Au sNN = 130 GeV/c

Data quality: 

Displaced vertices Ks  p+ + p- L  p + p- X-  L + p- W-  L + K- Data quality

Global Observables: 

Global Observables • WHAT ? * dNch/d, dET/d, <pT> * Reflect the conditions of the system well after freeze-out, after resonance decays • WHY ? * “Easy” measurements * Constrain models * Initial Conditions

Initial Conditions: 

Initial Conditions centrality defined as percentile of tot Npart, Ncoll, b • WHAT ? Impact Parameter “Spectators” “Spectators” “Participants”

Slide14: 

Centrality determination Use combination of Zero Degree Calorimeters Beam-Beam Counters (sensitive to 92% of geom) to define centrality classes Glauber modeling to extract N-participants 0-5% 5-10% 10-15% 15-20%

RHIC : dNch/d : 

PHOBOS: ||<1 , 1%? 2 layers of Si detectors close to vertex (B=0) dNch/d = 555 ± 12 ± 35 (6% most central) PRL dNch/d = 579 ± 1 ± 22 (6% most central) PHENIX: ||<0.35 ,  = 90o 2 layers of PC at 2.5 and 5 m from vertex (B=0) dNch/d = 622 ± 1 ± 41 (5% most central) STAR: ||<1.8 ,  = 2 Tracking in TPC, pt>100 MeV (B#0) dNch/d = 567 ± 1 ± 38 (5% most central) BRAHMS ||<4.7 Si strips, scintillators and Cherenkov counters dNch/d = 553 ± 1 ± 36 (5% most central) RHIC : dNch/d

Slide16: 

PHENIX preliminary Centrality Dependence : Comparison to CERN results

Slide17: 

Particle production mechanism (I) dNch/dh per participant increases vs Npar In contrast with EKRT saturation model Similar to HIJING (although data ~15% higher)

Slide18: 

Particle production mechanism (II) A. Capella, D. Sousa, nucl-th/0101023 BUT: • Capella, D. Sousa, nucl-th/0101023, No hard component. Soft processes scale with Ncoll • Kharzeev, Nardi, Phys. Lett.B507 (2001) 121 Saturation model agrees with data

Slide19: 

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

Slide20: 

Energy density a la Bjorken: From SPS to RHIC: at least 70% increase in e

Slide21: 

Transverse energy per charged particle dET /dNch independent of sNN dET /dNch independent of Npart • Consistent with very moderate increase of <pT> from AGS to RHIC: NA49 <pT>h- = 385 MeV RHIC <pT> 450 MeV • UA1: <pT> =392 MeV dET / dNch = 0.9

Slide22: 

dN/dh: Predictions * PHOBOS: 14 % increase in dNch/d from sNN = 130 to 200 GeV. * Dramatic increase predicted by HIJING with quenching not observed

Global Observables: Summary : 

Global Observables: Summary • ET per Nch independent of centrality and of energy • Systematic study of dET /dh and dNch/dh vs. Npart: * Stronger increase than at the CERN SPS * Evidence for role of hard processes ? • dNch/dy , dET/dy and Bjorken energy density ~70% higher in central Au+Au collisions at RHIC at ÖsNN=130 GeV compared to Pb+Pb collisions at CERN SPS at ÖsNN= 17.2 GeV • Very moderate increase O(20%) of <pT> between SPS and RHIC • Increase of particle production predicted by HIJING with quenching not observed.

What I will not show you : 

What I will not show you • Particle ratios and chemical equilibrium • Particle spectra  radial flow and thermal freeze out • Stopping and transparency: is the system net-baryon free at mid-rapidity? • Elliptic flow • HBT • Stangeness and multi-strangeness production

Physics accessible through e.m. probes (I): 

Physics accessible through e.m. probes (I) • Thermal Radiation a) Dileptons (e+e -, + -) * Large mfp no final state interaction carry information from place of creation to detectors. * Production rate strongly increasing with T and . most abundantly produced at the early stages. Best probes to look for thermal radiation from QGP: q q  *  l + l - HG: + -  *  l + l - b) Photons * Same underlying physics but much higher background less sensitivity

Physics accessible through e.m. probes (II): 

Physics accessible through e.m. probes (II) • Chiral Symmetry Restoration * Best candidates: -meson decay combined study of   l+ l- and   K+ K- • Strangeness Enhancement *  meson yield measured via its leptonic decay channels • Charm Enhancement * semileptonic decays of charmed mesons • Suppression of cc bound states * J/, ’…  e+e -, + - one of the earliest QGP signatures

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: 

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 masses or line broadening?

Quark – Hadron Duality: 

Quark – Hadron Duality In-medium + - ann. rates  perturbative qbarq ann. rates quark – hadron duality down to m ~ 0.5 GeV/c2 R. Rapp Thermal radiation from the plasma?

Low-Energy Run: 

Low-Energy Run At 160 GeV/u Baryon density is the dominant factor for dropping masses and for spectral shape broadening At lower energies B Softest point (P/) of equation of state occurs at 30 GeV/u Lowest pressure gradients Larger lifetimes

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

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 melt • One of the first observations: * J/ suppression in 200 A GeV S-Au collisions * Alternative explanation: absorption in nuclear medium: J/ + N  DD abs ~ 6mb • Anomalous suppression in Pb-Pb collisions

Anomalous J/y suppression in Pb-Pb collisions: 

Anomalous J/y suppression in Pb-Pb collisions Normal nuclear absorption: abs = 6.4 ± 0.8 mb NA50 Anomalous absorption in Pb-Pb for ET > 40 GeV or Npart >100 or b < 8fm

J/y suppression: Evidence of deconfinement ?: 

J/y suppression: Evidence of deconfinement ? PLB 477 (2000) 28 Conventional models ruled out Two-step pattern: successive melting of charmonium states c (binding energy  250 MeV) and J/ (650 MeV) NA50

Jet Quenching at RHIC?: 

Jets and minijets expected to contribute to particle production at RHIC energies Jet Quenching at RHIC?

pT Distributions in pp collisions: 

pT Distributions in pp collisions Data very well described by power law: pp = d2N/dpt2 = A (p0+pt)-n Scale to central (semi-inclusive) collisions: YAA (b) = pp TAA(b) = Ypp Ncoll  Nuclear modification factor:  Interpolate existing data to sNN = 130 GeV/c Scale to minimum bias (inclusive) AA collisions: AA = A2 pp

pt – distributions in AA collisionsat CERN : 

pt – distributions in AA collisions at CERN Known nuclear effects: Low pT : soft processes  Npart dominate R  Npart / Ncooll ~ 0.2 High pT : broadening due to rescattering (Cronin effect)  R > 1.

Charged pT Spectra: 

Charged pT Spectra

pT Spectra: comparison to pp : 

Yield in central collisions is significantly reduced relative to scaled pp yield pT Spectra: comparison to pp

Central to pp ratio: 

Ratio at high pT significantly below 1 o suppression more pronounced ? Pb-Pb data from CERN SPS show no such suppression ISR  data show moderate Cronin type enhancement Central to pp ratio

Central to Peripheral Ratio: 

Central to Peripheral Ratio Same behavior observed in the ratio of central to peripheral collisions Ncoll 10% central = 905 ± 96 Ncoll 60-80% peripheral = 20 ± 6

Slide44: 

Good agreement with peripheral collisions Calculations: X.N. Wang Comparison to Theory p-QCD over estimates the cross-section for p0 at least a factor of 5 Consistent with parton energy loss of dE/dx = 0.25 GeV/fm Red: “Cronin” + shadowing Green: dE/dx = 0.25GeV/fm Black : No nuclear effect Data:

Summary: 

Summary Outstanding start-up of RHIC machine and experiments A strong field: 15 years of AGS and SPS acievements A blossoming present with RHIC A bright future with RHIC and LHC Physics highlights: SPS: * Anomalous J/ suppression * Enhancement of low-mass dileptons * Enhancement of multistrange particles RHIC: * suppression of high pT particlesfor hard Very intriguing results. All consistent with QGP formation Stay tuned for RHIC run-2 data