Transcript Relativistic Heavy-Ion Physics: Experimental Overview SPSC @ Villars Villars-sur-Ollon September 22-28, 2004 Itzhak Tserruya RHIC Run-1 to Run-4 History Ldt s1/2 [GeV ] Run Year Species Run-1 Au+Au 1 mb-1 Au+Au 24 mb-1 Run-2 2001/2002 p+p Run-3 2002/2003 p+p Run-4 2003/2004 d+Au Au+Au Au+Au barn 0.15 pb-1 2.74 nb-1 0.35

Relativistic Heavy-Ion Physics:
Experimental Overview
SPSC @ Villars
Villars-sur-Ollon
September 22-28, 2004
Itzhak Tserruya
RHIC Run-1 to Run-4 History
Ldt
s1/2 [GeV ]
Run
Year
Species
Run-1
2000
Au+Au
130
1 mb-1
Au+Au
200
24 mb-1
Run-2
2001/2002
p+p
Run-3
2002/2003
p+p
Run-4
2003/2004
200
d+Au
200
Au+Au
Au+Au
barn
0.15 pb-1
200
2.74 nb-1
0.35 pb-1
200
62
Physics delivered
at ~all scales:
: Multiplicity
millibarn: Flavor
yields
microbarn: Charm
241 mb-1
9 mb-1
nanobarn: Jets
picobarn: J/Psi
 Most of the results presented here are from run 1 and 2, some from run 3
 Over 75 papers published in refereed journals, 55
55 of
ofthem
themininPRL.
PRL
 Impossible to review the wealth of results of the last four years in one single talk.
Will have to make choices.
2
Outline
News and Highlights
Global event characterization:
 Global observables
 Flow
 Chemical and thermal Equilibrium
Penetrating probes
 High pT suppression
 Jets
 Baryon puzzle
 Charm and charmonium
 Dileptons
 Photons
Outlook
3
Geometry of Heavy Ion Collisions
Non-central Collisions
Reaction plane
Nparticipants: number of incoming nucleons in the overlap region
Nbinary: number of inelastic nucleon-nucleon collisions
Centrality (a measure of the impact parameter) is defined as
4
percentile of the total cross section
Binary or Ncoll Scaling
Particle production via rare processes expected to
scale with Ncoll, the number of binary nucleonnucleon collisions
b
TA (b)    A (b, z )dz
b
Nuclear Thickness Function
If Nucleus " A" has A constituen ts
and Nucleus " B" has B constituen ts
which interact with cross section  INT
the TOTAL cross section  AB is :



 b
 b 
TAB (b)   TA ( s  )TB ( s  ) d s
2
2
Nuclear Overlap Function
 Test this on various rare processes

 AB   d 2b 1  e   INTTAB (b )   INT  d 2bTAB (b)
 A  B   INT for " small"  INT
5
Global
Observables
6
Global Observables
PHENIX preliminary
PHENIX preliminary
Energy density using the Bjorken estimate:
ε = dET / dy 1/πR2 1/τ
 ln dependence both of ET and Nch on
√sNN all the way from SIS up to RHIC
energies
Au-Au: πR2 = 139 fm2
τ [fm]
ε >> εc at all energies
ε [GeV/fm3]
τ [fm]
ε [GeV/fm3]
SPS
RHIC
LHC
1.0
0.3
0.1
2.5
17
90
1.0
2.5
1.0
5.0
1.0
9.0
7
<dET/dη> / <dNch/dη>
The ratio <ET>/<Nch:
Is
of of
centrality
isindependent
independent
centrality
– Still a puzzle.
– Since trigger and centrality
related uncertainties cancel out,
the flatness of the curves is quite
a precise statement.
is remarkably
remarkably constant
Is
constant
– increases by ~20% from 19.6 GeV to
130 GeV and stays the same between
130 GeV and 200 GeV
– At SPS and RHIC energies, the
available energy is invested more in
the creation of particles than in
increasing their momentum.
8
Flow
9
Flow: Evidence of Pressure and
Collective Effects
Reaction plane
Origin: In non-central
collisions, the pressure
converts, through multiple
scattering, the initial spatial
anisotropy (almond shape of
overlap region) into momentum
anisotropy
Collective effect.
Absent in pp collisions
v2: 2nd harmonic Fourier coefficient in azimuthal distribution of
particles with respect to the reaction plane
d 2N
 1  2v2 ( pT ) cos(2 )
ddpT
10
V2 at low pT
Flow hierarchy:
the lower the mass,
the higher the pT
v2(π)>v2(K)>v2(p)>v2(Λ)
Hydro works  Early thermalization
At low pT, hydrodynamic models reproduce well v2 values of various particles.
Calculations assume early thermalization, ideal fluid expansion, EOS consistent
with LQCD including a phase transition and sharp kinetic freeze-out at 120 MeV
11
V2 at high pT
The flow pattern is more complicated at high pT:
 v2(baryon) > v2(meson)
 v2 saturates at high pT
12
Simple v2 behavior at the quark level
The complicated flow pattern of hadrons becomes
very simple with the scaling behavior predicted by
quark recombination models:
v2 → v2 / n, pT → pT / n, n = number of valence quark
13
v2 at lower energies
Integrated v2
v2 slope
RHIC
SPS
AGS
 v2 larger at RHIC than at SPS
 quantified by the slope of v2/ε below pT = 1 GeV.
 Smooth increase of v2 from AGS  SPS  RHIC.
 Hydrodynamic calculations reproduce the data both at RHIC and SPS.
14
Equilibrium
Chemical freeze-out:
inelastic scattering stops  particle yields frozen
Thermal (or kinetic) freeze-out:
elastic scattering stops  particle spectra frozen
15
Evidence for equilibrated final state
Assume distributions given by one temperature and
one baryon chemical potential:
dn ~ e ( E μ) / T d 3 p
Two particle ratios are enough to determine both μ and T.
Then can predict all other particle ratios.
Observed hadron ratios in agreement with statistical
thermal model ratios, including multi-strange particles.
As observed at lower energies at the AGS and SPS.
16
Locate chemical freeze-out on phase
diagram
RHIC
Chemical freeze-out at RHIC and SPS
occur at/near the deconfinement
boundary of lattice QCD
SPS
Antiproton/proton ratio
AGS
RHIC
SPS
 RHIC very close
to the early universe
AGS
17
Thermal
Equilibrium
18
Identified Particle: (I) Spectra
Central
Low pT slopes
increase with
particle mass.
Proton and antiproton yields equal
the pion yield at
high pT.
Peripheral
Mass dependence
is less pronounced
Similar to pp
19
Identified Particle: (II) average pT
 Increase from peripheral to mid-central, and then saturate from
mid-central to central for all particle species.
 Clear mass dependence.(consistent with hydro picture)
radialexpansion.
expansion
 Indicative of radial
20
Example of Blast Wave Fits
Ref : Sollfrank, Schnedermann, Heinz, PRC48(1993) 2462.
R
 p sinh    m T cosh  
1 dN
 K1 

 A  f (r)rdrmT I0  T
0
m T dmT
Tfo
 Tfo  

boost: (r) = tanh bt  r/R
linear velocity profile bt(r) = btr
1
Parameters:
•
•
normalization A
freeze-out temp. Tfo
•
surface velocity bt
Fit |mT –m0| < 1GeV
and extrapolate
AuAu 200 GeV
F meson described
by same Tfo , bT
 Hydrodynamics describes all pT spectra up to 2 GeV/c.
21
Thermal freeze-out
Centrality dependence
Energy dependence
SPS
RHIC
AGS
 Collective expansion velocity increases
with centrality.
 At most central collisions <βT> ≈ 0.6c.
 Consistent with increase of <pT> with
centrality.
 Small and smooth increase of
mean transverse expansion velocity
as a function of beam energy
22
Intermediate Summary
Matter produced in relativistic heavy-ion collisions
characterized by:
– High energy density
– Strong collective flow
– Early thermalization
23
Penetrating Probes
“Penetrating probes” provide very sensitive diagnostic tools of the
high density matter created in ultra-relativistic heavy-ion collisions
Two types of “Penetrating probes”:
a) Probes created at early stage which propagate through, and are
modified by, the medium.
* QCD hard scattering probes:
 jet quenching  suppression of high pT hadrons
 J/ suppression
b) e.m. probes (real or virtual photons) created inside the medium
* Large mfp  no final state interaction
carry information from place of creation to detectors.
 low-mass e+e- pairs
 real photons
24
High pT suppression
25
Jets: A New Probe For High Density Matter
•
Jets from hard scattered quarks:
schematic view of jet production
- produced very early in the collision (τ <1fm/c)
AA
pp
- expected to be significant at RHIC
leading particle
•
In the colored medium quarks
radiate energy (energy loss ~GeV/fm)
modify jet shape.
q
q
leading particle
26
RHIC events
Au-Au
p-pcentral
collision
collision
at √s =at200
√sNN
GeV
= 200 GeV
STAR
PHENIX
STAR
27
Jets: A New Probe For High Density Matter
•
Jets from hard scattered quarks:
schematic view of jet production
- produced very early in the collision (τ <1fm/c)
AA
pp
- expected to be significant at RHIC
leading particle
•
•
In the colored medium quarks
radiate energy (energy loss ~GeV/fm)
modify jet shape.
Not possible to observe jets directly in
RHIC due to the large particle multiplicty.
q
q
leading particle
leadingparticles
particles
Identify jet and its possible modifications through leading
or correlations between the leading particles.
• Decrease their momentum  Suppression of high pT particles
“Jet Quenching”
28
Nuclear modification factor
• Zero hypothesis: scale pp to AA with the number of nn collisions Ncoll:
d2NAA/dpTd (b) = Nbin /?σinelpp d2σpp /dpTd = TAA(b) d2σpp /dpTd
• Quantify “effect” with nuclear modification factor:
d2 N AA /dpTd
R AA (pt ) 
TAA d2 pp /dpTd
RAA
RAA = 1
RAA < 1
• If no “effect”:
RAA < 1 at low pT in regime of
soft physics
RAA = 1 at high-pT where hard
scattering dominates
• If “jet quenching”:
RAA < 1 at high-pT
29
π0 pT spectra at √sNN = 200 GeV
p-p
Au -Au
η=0
Η=
 Precision p-p data is essential
 p-p data very well described by power-law: 1/pt dN/dpt = A (p0+pt)-n and
30
by NLO pQCD
p0 yield in AuAu vs. pp collisions
70-80% peripheral
Ncoll =12.3 ± 4.0
Excellent agreement between measured π0’s in p-p and
measured π0’s in Au-Au peripheral collisions scaled by
the number of collisions over ~ 5 decades
0-10% central
Ncoll =975 ± 94.0
Central Au-Au collisions yield significantly
suppressed relative to scaled pp yield
31
High pT Suppression in Au-Au collisions !!
AA / pp ratio
Major RHIC discovery to date
PRL 91, 072301 (2003)
Central/peripheral ratio
• New physics made
accessible by RHIC’s high
energy and ability to produce
(copious) perturbative probes
Charged particles
Peripheral collisions look like pp.
Central collisions are strongly suppressed
Factor 5!
Same behavior observed in the ratio
of central to peripheral collisions
32
Suppression increases gradually with
increasing collision centrality
Nuclear
modification factor
RAA for π0 and
charged particles
in different
centrality ranges
in Au+Au
collisions at
200GeV
33
d-Au Control Experiment
PLB 561 (2003) 82-92
 No suppression in dAu
but Cronin enhancement
 Suppression in AuAu is due to the
medium produced in the collision
PRL 91 (072303) 2003
Pion suppression reproduced by models
with parton energy loss via induced gluon
radiation (jet quenching) e.g., Gyulassy,
Levai Vitev, PLB 538 (2002) 282.
dNg/dy≈1000 dE/dx≈14GeV/fm
34
Is it really unique at RHIC energies?
•
Previous measurements at
CERN see enhancement, not
suppression in RAA
Low pt : soft processes  Npart
R  Npart / Ncoll ~ 0.2
CERN WA98: Understood
enhancement from Cronin effect
High pt : broadening due to
rescattering (Cronin effect)
 R > 1.
But RCP shows suppression !!
35
Is it really unique at RHIC energies?
D. D’Enterria
Phys. Lett. B596(2004) 32
However, reevaluation of pp reference data show that:
RAA for Pb-Pb central is consistent with Ncoll scaling
RAA seem to exhibit some degree of suppression from peripheral to central collisions
Emphasizes the crucial role of precise pp reference data.
36
Jets
37
Azimuthal distribution in Au+Au
Au+Au peripheral
Au+Au central
pedestal and flow subtracted
082302(2003)
STARPRL
PRL 90,
90, 082302
Near-side: peripheral and central Au+Au similar to p+p
Strong suppression of back-to-back
jets in central Au+Au
Medium is opaque
38
Proton puzzle
40
Protons are not suppressed !!!
PHENIX PRL 91, 172301 (2003).
At pT > 2GeV in central Au+Au collisions
 pions are suppressed
 protons are not
protons
different production mechanism ?
pions
p/π Ratio
Peripheral:
• consistent with fragmentation
• p/π ≈ 0.25 at high pT as in pp
Central:
• p/π ≈ 1
• strong centrality dependence
41
It is not only the protons
Presented by M. Lamont (QM04)
baryon
meson
Two distinct groups in Rcp , i.e. meson and baryon, not
by particle mass.
The two groups separate at pT ~ 2 GeV/c and seem to
come together at ~5 GeV/c?
42
Quark recombination models
explain the data
R
Duke model,
cp
PRC 68, 044902 (2003)
p/p
Duke model,
PRC 68, 044902 (2003)
• Describe Rcp, particle ratios , spectra, v2
• Interesting prediction at high pT soon to be tested
43
Associated particles per trigger
Correlations with identified mesons
and baryons
pT trigger: 2.5 – 4.0 GeV
pT associated: 1.7 – 2.5 GeV
0.0 < Δφ < 0.94
• associated
partner equally
likely for trigger
baryons &
mesons
• no centrality
PHENIX
dependence
(within errors)
 If these correlations originate from jet fragmentation,
why the baryons are not suppressed as the mesons?
44
 A challenge for the recombination models.
J/ψ
45
J/ Suppression
• An “old” signature of deconfinement:
(Matsui and Satz PL B178, (1986) 416).
Suppression Mechanism
 At high enough color density, the J/ finds itself
enveloped by the medium.
 When screening radius < binding radius  J/
will dissolve (Debye screening)
 The rarity of charm quarks makes it unlikely
that they find each other at the hadronization stage
c
c
Perturbative Vacuum
c
c
Color Screening
• One of the first observations at CERN:
* J/ suppression in 200 A GeV S-Au collisions explained by
absorption in nuclear medium J/ + N  DD
abs ~ 4mb
• Anomalous suppression in Pb-Pb collisions at CERN
46
Anomalous J/ψ Suppression
σabs = 4.3 ± 0.3 mb
Absorption deduced from
systematic measurements
of pA at 450 GeV and S-U
at 200 A GeV.
Most updated NA50 results:
suppression beyond normal
absorption clearly visible.
In agreement with
prediction of J/ψ melting in
a partonic medium. But…
47
Lattice QCD: Charmonia states above Tc
spectral function
• New lattice QCD: hadrons
don’t all melt at Tc!
• Calculations indicate that
charmonia states J/ψ (and
also ηc ) survive as distinct
resonances up to T=1.6Tc
Asakawa & Hatsuda,
PRL92, 012001 (2004)
•
p,  survive as resonances
Schaefer & Shuryak,
PLB 356 , 147(1995)
48
J/ at RHIC: Prospects
 Suppression or enhancement?
• suppressed: because of Debye screening of the attractive potential
between c and c in the deconfined medium.
• enhanced: charm cross section at RHIC is much larger than at
SPS. The J/ melting mechanism could be compensated by
recombination or coalescence of cc as the medium cools down.
 Energy loss of charm quarks in the high density medium
J/ is becoming a complex observable.
Will require precise measurements of pp, pA and AA
The PHENIX experiment was specifically designed to
measure J/  e+e- at mid-rapidity and J/  m+ m- at
forward rapidities
49
J/Y @ RHIC: Establishing pp baseline
Run2 (2001-2002):
-1 minbias evt
● limited statistics (150 nb
● only one muon arm
●poor detector performance
BR.tot = 234  36  34  24 nb
Run3 (2002-2003):
-1 minbias evts
● 350 nb
● complete muon spectrometer
BR.tot = 159 nb  8.5 %  12.3 %
Run4 (2004):
-1 minbias evts
● 350 nb
Clear J/ψ signals seen in both central and muon
arms.
● analysis in progress
Resolutions in agreement with expectations.
Up to now, statistics do not allow to
distinguish between
parton distribution functions50
d+Au vs pp @ 200 GeV
Au
d
y
y<0 large x in Au (~0.09)
anti-shadowing region
y>0 small x in Au (~0.003)
shadowing region
Vogt, PRL 91:142301,2003
Kopeliovich, NP A696:669,2001
• weak shadowing at low x (y>0)
• weak nuclear absorption
need more statistics to discriminate models
51
J/ψ 
+
ee
in Au-Au @ RHIC
Run 4
2
Poor statistics N=10.8
Preview
 3.2 (stat)  3.8 (sys)
J/  e+ e-
J/  μ+ μIncl. systematic errors
p-p
90 % C.L.
Most probable
value
Ncoll scaling band
Expectation with abs =4.4 and 7.1 mb
Clear
J/50
signal
in both
central and in
muon
arms
x
higher
luminosity
Run4
from
a small fraction of data.
-1
(240 mb minimum bias events, 270 Tbytes)
52
Charm
53
How to measure open charm?

direct reconstruction of charm decays D0  K- p+

But very challenging
 will require vertex detector upgrade
K
0 < pT < 3 GeV/c, |y| < 1.0
d+Au minbias



D0
D0+D0
c
c
alternative but indirect
– charm semi leptonic decays
contribute to single lepton
and lepton pair spectra:
K
D

0
D K
0

 
D0  K 

e

D0 D0  e e K  K   e  e
D0 D0  e  m  K  K   e  m
D0 D0  m  m  K  K   m 54m
Open Charm (via single e) in AuAu
First charm “measurement” @130 GeV:
Cocktail method
g conversion
cocktail analysis of inclusive e± in AuAu @
√sNN=130 GeV
establish “cocktail” of e± sources
p0  gee
  gee, 3p0
(π0, h, photon conversions are directly measured)
– light hadron decays
– photon conversions
w  ee, p0ee
  ee, ee
excess above cocktail
– increases with pT
– attributed to charm decays
  ee
’  gee
main
systematic errors (band)
55
–pion spectra, ratio /p0, ratio conversion/Dalitz (material)  Need converter run
Open charm: baseline is p+p collisions
PHENIX PRELIMINARY
• Measure charm via semileptonic decay to e+ & e• π0, η, photon conversions
are measured and
subtracted
pp @ 130 GeV:
cc= 420  33  250 mb
pp @ 200 GeV:
cc= 709  85  332/281 mb
Fit p+p data to get the baseline for d+Au and Au+Au.
56
1/TAA
1/TABEdN/dp3 [mb GeV-2]
AA
1/TAB1/T
EdN/dp3 [mb GeV-2]
AA
1/TAAABEdN/dp3 [mb GeV-2]
1/T
EdN/dp3 [mb GeV-2]
1/T1/T
AA AB
1/TABEdN/dp3 [mb GeV-2]
3 [mb GeV
3 [mb
GeV-2] -2]
1/T1/T
ABEdN/dp
ABEdN/dp
1/T
Charm scales with Ncoll in AuAu
For all centralities
~minimum bias
58
Low-mass pairs
59
Physics accessible through dileptons
• Best probe of Chiral Symmetry Restoration
Chiral symmetry spontaneously broken in nature.
Quark condensate is non-zero:
< qbarq >  300 MeV3  0 at high T and/or high 
Constituent mass  current mass
Chiral Symmetry (approximately) restored.
Meson properties (m,) expected to be modified (?)
* Best candidate: -meson decay ( = 1.3fm/c)
• Dileptons (e+e -, m+m -): best probes to look for thermal radiation from
QGP: q q  g*  l + l - or
HG:
p+p -  g*  l + l •
F meson
* simultaneous measurement of   l+ l- and   K+ Kvery powerful tool to evidence in-medium effects
* strangeness enhancement
60
Low-mass Dileptons: Main CERES Result
Strong enhancement of low-mass e+e- pairs in A-A collisions
(wrt to expected yield from known sources)
Most updated CERES result
(from 2000 Pb run):
Enhancement factor (0.2 <m < 1.1 GeV/c2 ):
3.1 ± 0.3 (stat)
No enhancement in pp
nor in pA
61
In-medium modification of light vector meson
Interpretations invoke:
* p+p-  g*  e+e-
thermal radiation from HG
not enough to reproduce data
* in-medium modifications of :
- broadening  meson spectral shape
(Rapp and Wambach)
- dropping  meson mass
(Brown et al)
Connection to Chiral Symmetry Restoration? G.E Brown, C.H. Lee and M. Rho hep-ph/0405114
NA60 to continue and complete the program
62
Low-mass e+e- Pairs: Prospects at RHIC
R. Rapp nucl-th/0204003
 interpretation of SPS data rely on a high baryon
density at mid rapidity.
Baryon density is almost the same at RHIC and SPS
• Strong enhancement of low-mass
pairs persists at RHIC
63
Low and intermediate mass pairs at RHIC
Real and Mixed e+e- Distribution
Real - Mixed e+e- Distribution
e+e- from light
hadron decays
e+e- pairs (real)
e+e- pairs (mixed)
net e+e-
e+e- from
charm
(PYTHIA)
First look from low-luminosity Run-2: (m = 0.3 – 1.0 GeV):
Predictions: =
9.2 x 10-5
Measurements:
.2
5
2
13.4  7.2(stat)12
(
sys
)

10
[c
/ GeV ]
8.4
Problem: combinatorial background too high S/B  1/300
• Run4  resonance decays
• Run6-7 HBD upgrade
64
Photons
65
Direct Photons in AuAu: pQCD works at
high pT.
Au+Au 200 GeV/A: 10% most central collisions
PHENIX Preliminary PbGl / PbSc Combined
1 + (g pQCD direct x Ncoll) / g
phenix backgrd
Vogelsang NLO
1 + (g pQCD direct x Ncoll) / g
phenix backgrd
Vogelsang, m = 0.5,
2.0
1 + (g pQCD direct x Ncoll) / (g
phenix pp backgrd
x Ncoll)
p0 suppressed
g not suppressed
g & p0 both not suppressed
pT (GeV/c)
[g/p0]measured / [g/p0]background =
gmeasured/gbackground
Theory curves include PHENIX gexpected background calculation based on p0:
66
(g direct + gexp. bkgd.) / gexp. bkgd. = 1 + (gdirect/gexp. bkgd.)
Ncoll Scaling for Direct Photons
PHENIX Preliminary
Vogelsang
NLO
The direct photon yield in Au+Au is described
by pp NLO calculations scaled with Ncoll
67
Summary and Outlook
Matter produced in relativistic heavy-ion collisions characterized by:
– High energy density
– Strong collective flow
– Early thermalization
Medium effects:
– Modification of jet fragmentation
– J/ψ suppression
– Low-mass dilepton enhancement
Hints of deconfinement and chiral symmetry restoration. No
irrefutable evidence of and thermal radiation.
Not an ideal gas of free quarks and gluons. Looks more like a
strongly interacting QGP.
 A blossoming present with outstanding performance of RHIC
machine and experiments and results still coming out of SPS
 AApromising
promisingfuture
future with FAIR, LHC and RHIC-upgrades in the horizon.
…. and SPS ?
68