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/ suppression– low mass dilepton enhancement– high pT suppression
• Summary
Lattice QCD at Finite Temperature• Coincident transitions: deconfinement and chiral symmetry restoration
F. Karsch, hep-ph/010314
Critical energy density:4)26( CC T
TC ~ 175 MeVC ~ 0.7 GeV/fm3
Ideal gas (Stefan-Boltzmann limit)
B=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.
MeVqq 3)250(
0qq
4
Schematic Phase Diagram of Strongly Interacting MatterSchematic Phase Diagram of Strongly Interacting Matter
Baryonic Potential B [MeV]
T
em
pera
ture
T [
MeV
]
0
200
250
150
100
50
0 200 400 600 800 1000 1200
AGS
SIS
SPS
RHIC
quark-gluon plasma
hadron gas neutron stars
early universe
thermal freeze-outdeconfinementchiral restoration
Lattice QCD
atomic nuclei
P. Braun-Munzinger, nucl-ex/0007021
Test QCD under extreme conditions and in large scale systems
Search for deconfined QGP phase
SISAGS SPS RHICLHC
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 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
7
The RHIC Experiments
STAR
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)
100% 0 %
Participants
Spectators
Spectators
Charged-Particle Rapidity Distribution
BRAHMS (0-5%): Nch (||<4.7) = 3860 ± 300NA49 (0-5%): Nh
- (|y| < 3) = 695 ± 30
- Factor of 3 more particles produced at RHIC than at SPS - Wider distribution
Enhancement of particle production for central collisions at mid-rapidity.
Particle production scales with Npart at high rapidities ( >3).
h-
NA49
dn/dy
RHIC
SPS
BRAHMS
11
From SPS to RHIC :
* dNch/dy increases by
~70% at sNN
= 130 GeV
* dNch/dy increases by
~90% at sNN
= 200 GeV
ln(sNN
) dependence from
AGS to RHIC
sNN
Dependence of dNch/dy
AGS
SPSRHIC
12
Radial Flow
– Expansion of system due to pressure
– Heavier particles shifted to higher pT
– Observable: <T> 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
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%)
• 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) =200GeVAu+Au at sqrt(sNN) =200GeV
J. Burward-Hoy, QM2002
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
PHENIX:Freeze-out Temperature
Tfo = 110 23 MeV
Transverse flow velocity
T = 0.7 0.2 < T> ~ 0.5
Most central collisionsfor 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
T ~ 0.6
J. Burward-Hoy, QM2002
17
Mid-Rapidity mT spectra at SPS
M. van Leeuwen QM2002 (NA49)
NA57, H. Helstrup, this conference:
Tfo = 131 ± 10 MeV
<T> = 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
x
y
p
patan2cos2 vSecond Fourier coefficient v2:
Coordinate space: initial asymmetry
Momentum space: final asymmetry
multiple collisions (pressure)
py
px
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
20
Elliptic Flow
• SPS: v2 ~ 0.03
• RHIC: v2 ~ 0.055
Wetzler QM2002
E877: Phys.Lett.B474:27-32, 2000CERES: QM2001INPC 2001 nucl-ex/0109017 STAR: PRC66 (2002) 034904NA49 Preliminary
130 GeV data
21
Flow: Comparison of SPS and RHIC
• Radial Flow: pressure can build up over entire dynamics– <T> ~ 0.4 - 0.5 at SPS– <T> ~ 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 <T> more pressure at RHIC• Significantly larger v2 is evidence for early build-up of
pressure• According to hydrodynamic models early
thermalization at RHIC (~0.6fm/c - Heinz, Kolb
Nucl.Phys.A702:269-280,2002 )
22
Energy Density
Energy density a la Bjorken:
dy
dE
τπR
1ε T
2
fm/c 12.0τ
fm/c 1τ
A 1.18R
RHIC
SPS
1/3
38~6.0 GeV/fmfm/c
dET/dy ~ 720 GeV (S. Bazilevsky
QM2002, PHENIX PRELIMINARY)
35~1 GeV/fmfm/c
Estimate for RHIC:
23
Other Highlights of Program
• Global observables properties of collision dynamics, EOS
• Other probes for signatures of QGP– J/ suppression deconfinement– low mass dileptons chiral symmetry restoration
– high pT suppression density of produced medium and energy loss
24
J/ 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
25
J/ suppression: Evidence of deconfinement?L. Ramello, QM 2002NA50 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)
26
• possible signature of the deconfinement phase transition– J/ 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/ in p+p and d+Au to separate “normal” nuclear effects– shadowing– nuclear absorption in cold matter
• Jmeasurements in leptonic decay channels– J/ e+ e- and J/ in p+p at s = 200 GeV
– J/ e+ e- in Au+Au at sNN = 200 GeV
Charmonium (Jphysics at RHIC
(hep-ex/0307019)
(nucl-ex/0305030)
27
J/ Production at RHIC
normal nuclear absorption:
Pb+Pb at CERN SPS (NA50)
PHENIX, sNN
= 200 GeV • J/-Suppression maybe most compelling QGP evidence at CERN SPS
• Expectation at RHIC energies unclear0 cc pairs
produced per central Au+Au collision
– Possibly enhanced J/- production due to charm-coalescence
-
PLB477(2000) 28 normalized to PHENIX p+p measurement
28
coalescence model (Thews at al.)
y = 1.0
y = 4.0
statistical model (Andronic at al.)
absorption model (Grandchamp et al.)
Model comparisons
• models that predict enhancement relative to binary collision scaling are disfavored
• no discrimination between models that lead to suppression
29
Low-Mass e+e- pairs
No enhancement in pp and pA collisions
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)
30
Interpretations
scattering off baryons(Rapp, Wambach et al)
-meson broadening Dropping -meson mass(G.E. Brown et al)
annihilation: +- * e+e- (thermal radiation from HG)Cross section dominated by pole at the mass of the em form factor:
2222
42
m )m (m
m m)(F
Plus
or
Add
Onset of Chiral Symmetry Restoration?Dropping -meson mass
(Rapp, Wambach et al)
In-medium -meson broadening(G.E. Brown et al)d.o.f.
hadrons quarks
What happens as chiral symmetry is restored? Dropping mass or broadening (melting)?
32
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”
hadrons
q
q
hadrons leadingparticle
leading particle
schematic view of jet production
33
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
RAA (pT ) d2N AA /dpT d
TAA d2 NN /dpT d
<Nbinary>/inelp+p
NN cross section
34
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 dominatesRHIC: suppression dominates
RAA for 0
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
35
Jet Quenching ?• high pT suppression
reproduced by models with parton energy loss
• other explanations not ruled out, need to measure initial-state effects
without parton energy loss
with parton energy lossWang
Wang
Levai
Levai
Vitev
comparison with model calculations
with and without parton energy loss
Au+Au0+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).
36
RAA for 0 and charged hadrons
pp
AuAubinaryAuAuAA Yield
NYieldR
/
PHENIX AuAu 200 GeV0 data: nucl-ex/0304022, submitted to PRL.charged hadron (preliminary) : NPA715, 769c (2003).
• RAA is well below 1 for both charged hadrons and neutral pions.
• The neutral pions fall below the charged hadrons since they do not contain contributions from protons and kaons (will be discussed later).
Strong Suppression!
- Consistent observation by all 4 experiments in charged hadron measurement
PHOBOS, R. Nouicer, this conferenceBRAHMS, Z. Yin, this conference
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
Au+Au peripheral Au+Au central
pedestal and flow subtracted
Phys Rev Lett 90, 082302
?
38
d+Au
Au+Au
RAA vs. RdA for charged hadrons and 0
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/0306021submitted to PRL
PHOBOS, R. Nouicer, this conference
BRAHMS, Z. Yin, this conference
39
Azimuthal distributions
pedestal and flow subtracted
Near-side: p+p, d+Au, Au+Au similarBack-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
40
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?
41
Particle Species Dependence of High pT Suppression
No apparent proton suppression for 2-4 GeV/c – different production mechanism ?
PHENIX, nucl-ex/0305036
(Similar effect seen in STAR for vs. Kshort suppression)
peripheralbinaryperipheral
centralbinarycentral
NYield
NYield
//
42
Particle Composition at High pT
• p/ < 0.25 expected from jet fragmentation• observed 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)
• 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 WA98
46
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
WA98WA98
Hydro calculations :Prompt + QGP Mixed phase HG QGP dominates at high pT
Srivastava and Sinha nucl-th/0006018Srivastava and Sinha nucl-th/0006018
47
Direct Photons Direct Photons:– Photons not originating
from hadron decays like 0
all
direct+
decay
• Direct photon signal seen in Pb+Pb at s
NN=17.3 GeV
• Stronger signal expected at RHIC, because 0 suppressed by factor 5– Suppression appears to
be a final state effect– Direct photons not
affected by final state interactions
pQCD calculation for direct and 0
in p+p at s=200 GeV(Werner Vogelsang):
48
Direct Photon Search
• Au+Au at sNN
= 200 GeV
• No direct photon signal seen within errors
• With further analysis systematic errors will be reduced ...
preliminarypreliminary
preliminary
49
Azimuthal asymmtery (v2) at high pT
STAR
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 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
0 at sNN = 200 GeVnucl-ex/0304022, submitted to PRL
pp
AuAubinaryAuAuAA Yield
NYieldR
/
51
Particle Spectra Evolution“Peripheral
”
Particle
Physics
“Central”
Nuclear
Physics
52
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.
“PHENIX Preliminary” results on centrality-dependence consistent with PHOBOS data
Au + Au Experiment d + Au Experiment
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/ suppression: Evidence of deconfinement?
PLB 477 (2000) 28
NA50 preliminaryNA50 preliminary
L. Ramello, QM 2002
melting of charmonium states: c (binding energy 250 MeV)
and J/ (650 MeV)
55
Jet correlations: Au+Au vs. p+pSTAR PRL 90, 082302 (2003)
22 2 2( ) ( ) (1 cos(2 ))D Au Au D p p B v
Back-to-back jets are suppressed in central collisions!
near side
away side
peripheral central
Peripheral Au + Au
Central Au + Au
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
0-5%
10-15%15-20%
5-10%
PHENIX
57
Centrality Dependence: Comparison to Models
Saturation models reproduce the scaling with centrality and energy dependence!
dN
ch/d/
(0.5
Np
art)
Kharzeev & Levin, nucl-th/0108006Schaffner-Bielich et al, nucl-th/0108048
58
- Centrality Dependence of Pion Suppression -
• smooth increase of suppression with centrality
• neither binary or participant scaling
0 at sNN = 200 GeVnucl-ex/0304022, submitted to PRL
59
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
NA35 NA36
NA49
NA34/2HELIOS2
NA34/3HELIOS3
NA44
NA45CERES
NA38
NA50
NA60
WA80
WA98
WA85
WA97
NA57
NA52
WA94SO
Pb
multistrangephotonshadrons
dimuons
dielectrons
1986
1994
2000
hadrons
strangeletshadrons
hadronsdimuons
1992
2003
Carlos Lourenco QM01Carlos Lourenco QM01
60
Color Glass CondensateAlternate 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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