Relativistic Heavy Ion Physics: An Experimental Review

1
Relativistic Heavy Ion Physics An Experimental
Review
Saskia Mioduszewski
22 July 2003
2
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

3
Lattice QCD at Finite Temperature

• Coincident transitions deconfinement and chiral
  symmetry restoration

Ideal gas (Stefan-Boltzmann limit)
F. Karsch, hep-ph/010314
(mB0)
Critical energy density
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.
TC 175 MeV ? eC 0.7 GeV/fm3
4
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
5
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

6
History of High-Energy AB Beams

• BNL-AGS mid 80s, early 90s
• OA, SiA 15 AGeV/c ?sNN 6 GeV
• AuA 11 AGeV/c ?sNN 5
  GeV
• CERN-SPS mid 80s, 90s
• OA, SA 200 AGeV/c ?sNN 20 GeV
• PbA 160 AGeV/c ?sNN 17
  GeV
• BNL-RHIC early 00s
• AuAu
  ?sNN 130 GeV
• AuAu, pp, dAu ?sNN 200
  GeV

7
The RHIC Experiments
8
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

9
AA collisions are not all the same

• Nuclei are extended objects
• Impact parameter
• Number of participants
• Centrality
• ( from total inelastic cross-section)

10
Charged-Particle Rapidity Distribution
RHIC
BRAHMS
SPS

• Enhancement of particle production for central
  collisions at mid-rapidity.
• Particle production scales with Npart at high
  rapidities (h gt3).

11
ÖsNN Dependence of dNch/dy

• ? 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

12
Radial Flow

• Expansion of system due to pressure
• Heavier particles shifted to higher pT

Observable ltbTgt from slopes of mT spectra as a
function of mass Spectra can be described by
hydrodynamic models for pTlt 2-3 GeV/c and
mid-peripheral to central events
13
Single Particle Spectra (low pT)

• Decreasing slope for increasing particle mass
  and centrality

T. Ullrich QM2002
14
Single Particle Spectra for most central events
(0-5)
J. Burward-Hoy, QM2002
PHENIX Preliminary
PHENIX Preliminary
AuAu at sqrt(sNN) 200GeV
AuAu at sqrt(sNN) 200GeV

• proton yield pion yield _at_ 2 GeV
• consistent with hydrodynamic model calculations
  (e.g. comparison to 130 GeV data -
  Teaney, Lauret, Shuryak nucl-th/0110037)

15
Mean Transverse Momentum vs. Npart
J. Burward-Hoy, QM2002
closed symbols 200 GeV
open symbols 130 GeV
ltpTgt increases with Npart and particle mass,
indicative of radial expansion Relative increase
with Npart greater for (anti)p than for ?, K
16
Hydrodynamic Model Fit to the Spectra
J. Burward-Hoy, QM2002
Most central collisions for 200 GeV data
PHENIX Freeze-out Temperature Tfo 110 ? 23
MeV Transverse flow velocity bT 0.7 ? 0.2 ? lt
bTgt 0.5
AuAu at sqrt(sNN) 200GeV
Ref E. Schnedermann, J. Sollfrank, and U.
Heinz, Phys. Rev. C 48, 2462 (1993)
STAR Tfo 100 MeV ?bT? 0.6
17
Mid-Rapidity mT spectra at SPS
M. van Leeuwen QM2002 (NA49)
NA57, H. Helstrup, this conference Tfo 131
10 MeV ltbTgt 0.47 0.02
18
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
19
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).

20
Elliptic Flow
130 GeV data
Wetzler QM2002

• SPS v2 0.03
• RHIC v2 0.055

E877 Phys.Lett.B47427-32, 2000 CERES
QM2001 INPC 2001 nucl-ex/0109017 STAR PRC66
(2002) 034904 NA49 Preliminary
21
Flow Comparison of SPS and RHIC

• Radial Flow pressure can build up over entire
  dynamics
• ltbTgt 0.4 - 0.5 at SPS
• ltbTgt 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 ltbTgt ? more pressure at RHIC
• Significantly larger v2 is evidence for early
  build-up of pressure
• According to hydrodynamic models ? early
  thermalization at RHIC (t0.6fm/c - Heinz, Kolb
  Nucl.Phys.A702269-280,2002 )

22
Energy Density
Energy density a la Bjorken
Estimate e for RHIC
dET/dy 720 GeV (S. Bazilevsky QM2002, PHENIX
PRELIMINARY)
23
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

24
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 lt binding radius
  ? J/? will dissolve

Observation Anomalous suppression in Pb-Pb
collisions beyond normal nuclear absorption
?abs 4-6 mb
25
J/y suppression Evidence of deconfinement?
L. Ramello, QM 2002
NA50 Preliminary
Suppression increasing with centrality
(discontinuities?) Exceeds normal nuclear
absorption (as measured in pA) Many models
exist (hadronic and QGP) data consistent with
suggested QGP signature (Matsui, Satz, Kharzeev)
26
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 pp and dAu 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 pp at ?s 200
  GeV
• J/Y ?? e e- in AuAu at ?sNN 200 GeV

(hep-ex/0307019)
(nucl-ex/0305030)
27
J/Y Production at RHIC

• J/Y-Suppression maybe most compelling QGP
  evidence at CERN SPS
• Expectation at RHIC energies unclear
• ?10 cc pairs produced per central AuAu
  collision
• Possibly enhanced J/Y- production due to
  charm-coalescence

PHENIX, ?sNN 200 GeV
-
PLB477(2000) 28 normalized to PHENIX pp
measurement
28
Model comparisons

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

29
Low-Mass ee- pairs
Main CERES Result Strong enhancement of low-mass
pairs in A-A collisions (wrt to expected yield
from known sources)
Enhancement factor (.25 ltmlt.7GeV/c2)
2.6 0.5 (stat) 0.6 (syst)
30
Interpretations
Add
?? annihilation ??- ? ? ? ee- (thermal
radiation from HG)
Cross section dominated by pole at the ? mass of
the ? em form factor
Plus
or
31
Onset of Chiral Symmetry Restoration?
Dropping ?-meson mass
In-medium ?-meson broadening
(Rapp, Wambach et al)
(G.E. Brown et al)
What happens as chiral symmetry is restored?
Dropping mass or broadening (melting)?
32
Fate of Hard Scattered Partons in AuAu 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

33
Nuclear Modification Factor RAA
Nuclear Modification Factor
ltNbinarygt/sinelpp
NN cross section

• in absence of nuclear effects
• RAA lt 1 at low pT (soft physics regime)
• RAA 1 at high pT (hard scattering regime)
• suppression (enhancement, e.g. Cronin effect)
• RAA lt 1 (gt 1) at high pT

34
RAA for p0

• By definition, processes that scale with Nbinary
  will produce RAA1.
• RAA is what we measure divided by what we expect.

Nbinary-scaling
RAA is lt 1 at RHIC, but gt 1 at SPS SPS Cronin
effect dominates RHIC suppression dominates
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
35
Jet Quenching ?

• high pT suppression reproduced by models with
  parton energy loss
• other explanations not ruled out, need to measure
  initial-state effects

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).
AuAu?p0X at ?sNN 200 GeV
36
RAA for p0 and charged hadrons
PHENIX AuAu 200 GeV p0 data nucl-ex/0304022,
submitted to PRL. charged hadron (preliminary)
NPA715, 769c (2003).
37
Azimuthal distributions in AuAu
Near-side peripheral and central AuAu similar
to pp
Strong suppression of back-to-back correlations
in central AuAu collisions
38
RAA vs. RdA for charged hadrons and p0
Initial State Effects Only
PHENIX (dAu) hep-ex/0306021 submitted to PRL
Initial Final State Effects

• No Suppression in dAu, 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

39
Azimuthal distributions
Near-side pp, dAu, AuAu similar Back-to-back
AuAu strongly suppressed relative to pp and dAu
Suppression of the back-to-back correlation in
central AuAu is a final-state effect
40
High pT Measurements at RHIC

• dAu collisions
• No suppression at high pT
• Away-side jet strength consistent with pp
  collisions
• Peripheral AuAu collisions
• Hadron yields consistent with Nbinary-scaled
  yields in pp collisions
• Away-side jet strength consistent with pp
  collisions
• Central AuAu collisions
• Hadrons are suppressed at high pT (up to 10
  GeV/c)
• Away-side jet disappears
• Particle Composition in Central AuAu collisions
  What is happening with the protons?

41
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)
42
Particle Composition at High pT

• p/p lt 0.25 expected from jet fragmentation
• observed p/p 0.4 in peripheral, 1 in central
• protons from non-fragmentation sources ?

nucl-ex/0305036
43
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

44
Extra Slides
45
Direct Photons (I)
WA98

• 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)

46
Direct Photons (II)
WA98

• Comparison to scaled pA similar spectrum
• but factor of 2 enhanced yield in Pb-Pb,
• again 1? effect.
• pQCD underpredicts direct photon yield

47
Direct Photons

• Direct Photons
• Photons not originating from hadron decays like
  p0?gg
• gall gdirect gdecay
• Direct photon signal seen in PbPb at ?sNN17.3
  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 pp at
?s200 GeV (Werner Vogelsang)
48
Direct Photon Search

• AuAu at ?sNN 200 GeV
• No direct photon signal seen within errors
• With further analysis systematic errors will be
  reduced ...

49
Azimuthal asymmtery (v2) at high pT
Finite v2 up pT 10 GeV Hydrodynamics up to pT
2-3 GeV Jets correlated to reaction plane?
50
Neutral Pion Production in central and peripheral
AuAu collisions

• reference pp data with same detector
• binary scaling in peripheral AuAu
• suppression factor
• 5 in central AuAu

Binary scaling
1/5
Participant scaling
p0 at ?sNN 200 GeV nucl-ex/0304022, submitted
to PRL
51
Particle Spectra Evolution
52
Centrality Dependence
Au Au Experiment
d Au Experiment

• Dramatically different and opposite centrality
  evolution of AuAu experiment from dAu control.
• High pT hadron suppression in AuAu is due to a
  final state effect.

53
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?
54
J/y suppression Evidence of deconfinement?
PLB 477 (2000) 28
L. Ramello, QM 2002
melting of charmonium states ?c (binding
energy ? 250 MeV) and J/? (650 MeV)
NA50 preliminary
55
Jet correlations AuAu vs. pp
Peripheral Au Au
STAR PRL 90, 082302 (2003)
Central Au Au
Back-to-back jets are suppressed in central
collisions!
56
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
57
Centrality Dependence Comparison to Models
Kharzeev Levin, nucl-th/0108006 Schaffner-Bielic
h et al, nucl-th/0108048
dNch/dh/(0.5Npart)
Saturation models reproduce the scaling with
centrality and energy dependence!
58
- 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
59
The SPS Experiments

• 1986 - 1987 Oxygen _at_ 60 200 GeV/nucleon
• 1987 - 1992 Sulphur _at_ 200 GeV/nucleon
• 1994 - 2000 Lead _at_ 40, 80 158 GeV/nucleon
• 2002 - 2003 Indium and Lead _at_ 158 GeV/nucleon

And proton beams for pp and pA reference studies
Carlos Lourenco QM01
60
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.

High x
Low x
Color Glass Condensate hep-ph/0210033 Gribov,
Levin, Ryshkin, Mueller, Qiu, Kharzeev,
McLerran, Venugopalan, Balitsky, Kovchegov,
Kovner, Iancu