Link to cosmology
A QCD/QGP/RHIC primer
The discovery of the sQGP
Towards the most fundamental questions
High Energy Heavy Ion Physics
Quo Vadis ?
Rene Bellwied
Wayne State University
(bellwied@physics.wayne.edu)
Motivation for Relativistic Heavy
Ion Collisions
Two big connections: cosmology and QCD
Going back in time…
Age
0
10-35 s
Energy
1019 GeV
1014 GeV
Matter in universe
grand unified theory of all forces
1st phase transition
(strong: q,g + electroweak: g, l,n)
10-10 s
102 GeV
2nd phase transition
(strong: q,g + electro: g + weak: l,n)
10-5 s
0.2 GeV
3 min.
0.1 MeV
RIA & FAIR
RHIC, LHC &(strong:hadrons
FAIR + electro:g + weak: l,n)
6*105 years 0.3 eV
Now
(15 billion years)
3rd phase transition
3*10-4 eV = 3 K
nuclei
atoms
Evolution of Forces in Nature
Connection to Cosmology
• Baryogenesis ? Separation of Matter and Antimatter – can it
happen at the phase transition ?
• Dark Matter Formation ? – can it happen at the phase
transition ?
• Dark Energy Formation – can it happen at the phase
transition ?
• Is matter generation in cosmic medium (plasma) different
than matter generation in vacuum ?
• Can fluctuations at the phase transition explain an
anisotropic matter distribution in the universe ?
Sakharov (1967) – three conditions
for baryogenesis
• Baryon number violation
• C- and CP-symmetry violation
• Interactions out of thermal equilibrium
•
Currently, there is no experimental evidence of particle interactions where the
conservation of baryon number is broken: all observed particle reactions have equal
baryon number before and after. Mathematically, the commutator of the baryon number
quantum operator with the Standard Model Hamiltonian is zero: [B,H] = BH - HB = 0.
This suggests physics beyond the Standard Model
•
The second condition — violation of CP-symmetry — was discovered in 1964 (direct
CP-violation, that is violation of CP-symmetry in a decay process, was discovered
later, in 1999). If CPT-symmetry is assumed, violation of CP-symmetry demands
violation of time inversion symmetry, or T-symmetry. Under investigation
•
The last condition states that the rate of a reaction which generates baryon-asymmetry
must be less than the rate of expansion of the universe. In this situation the particles
and their corresponding antiparticles do not achieve thermal equilibrium due to rapid
expansion decreasing the occurrence of pair-annihilation.
A mass problem of universal proportion
• The stars and gas in most galaxies
move much quicker than expected
from the luminosity of the galaxies.
• In spiral galaxies, the rotation curve
remains at about the same value at
great distances from the center (it is
said to be ``flat'').
• This means that the enclosed mass
continues to increase even though
the amount of visible, luminous
matter falls off at large distances
from the center.

Something else must be adding to the gravity of the galaxies
without shining. We call it Dark Matter !
According to measurements it accounts for > 80% of the mass
in the universe.
The cosmic connection of RHI physics
Witten’s ‘Cosmic Separation of phases’ (Phys.Rev.D 30 (1984) 272)
basic parameter: mass
Originally: strange quark matter was a prime candidate for
dark matter (as recent as SQM 2003)
Dark Matter vs. Luminous Matter distribution
Bullet Cluster, 3.4 Billion Lightyears from Earth
X-ray image vs. gravitational lensing
The universe is accelerating….
Based on supernovae measurements
You need DARK ENERGY as an explanation (!?)
Dark Energy does not kick in at the
time of the Big Bang !
The cosmic connection of RHI physics
Let’s understand mass generation in the luminous matter
What do we know about quark masses ?
Why are quark current masses so
different ?
There is no answer to this
questions.
There likely will be no answer to
this question !
Nature’s constants:
-speed of light, electric charge,
quark current masses (?)
Very little is known, very little can be explained
Standard model is symmetric
All degrees of freedom are massless
Electro-weak symmetry breaking
via Higgs field (Dm of W, Z, g)
Mechanism to generate current quark
masses
(but does not explain their magnitude)
Chiral symmetry breaking
via dynamical quarks
Mechanism to generate constituent
quark masses
(but does not explain hadronization)
We can’t answer the question of mass
generation at the most fundamental level,
but can we answer the question of mass
generation at the nuclear level ?
Theory:
Quantum
Chromo
Dynamics
The fundamental problem: how is baryonic mass generated
Based on quark interactions (5+10+10 = 935 MeV/c2) ?
Theoretical and computational (lattice) QCD
In vacuum:
- asymptotically free quarks have current mass
- confined quarks have constituent mass
- baryonic mass is sum of valence quark constituent masses
Masses can be computed as a function of the evolving coupling
strength or the ‘level of asymptotic freedom’, i.e. dynamic masses.
But the universe was not a vacuum at the time of hadronization,
it was likely a plasma of quarks and gluons. Is the mass generation
mechanism the same ?
The main features of Quantum
Chromodynamics (QCD)
• Confinement
– At large distances the effective coupling between quarks is large, resulting in
confinement.
– Free quarks are not observed in nature.
• Asymptotic freedom
– At short distances the effective coupling between quarks decreases
logarithmically.
– Under such conditions quarks and gluons appear to be quasi-free.
• (Hidden) chiral symmetry
– Connected with the quark masses
– When confined quarks have a large dynamical mass - constituent mass
– In the small coupling limit (some) quarks have small mass - current mass
Analogies and differences between QED and QCD
to study structure of an atom…
electron
…separate constituents
nucleus
Imagine our understanding of atoms or QED if we
could not isolate charged objects!!
neutral atom
ToConfinement:
understandfundamental
the strong
force and the phenomenon of confinement:
& crucial (but not understood!) feature of strong force
- colored
objects (quarks)
have  energy
in normal
vacuum
Create and study
a system
of deconfined
colored
quarks
(and gluons)
quark-antiquark pair
created from vacuum
quark
“white” proton
(confined quarks)
Strong color field
“white” 0
“white”
proton
Force
grows
with separation(confined
!!!
quarks)
A mechanism of hadronization in vacuum:
String Fragmentation
High momentum current mass quark pair forms flux tube in a
collision = string of energy (string tension) i.e. dynamical quark field
which fragments into hadrons when string tension becomes too large.
Describes e+e- and p-pbar and p-p collisions well.
Hadronization in medium (i.e. during universe expansion) could be
different because medium might affect the mechanism.
The temperature dependent running
coupling constant as and its effect on
mass generation above Tc
O.Kaczmarek et al. (thermal mass, LQCD)
(hep-lat/0406036)
1.05 Tc
1.5 Tc
3 Tc
12 Tc
6 Tc
in an expanding system: interplay between
distance and temperature
Massive partons above Tc
e.g. P.Levai and U.Heinz
(hep-ph/9710463)
Lattice QCD:
Chiral Symmetry is restored at Tc
One goal: Proving asymptotic freedom
in the laboratory.
Nobel Prize 2005
D. Gross
H.D. Politzer
F. Wilczek
QCD Asymptotic Freedom (1973)
• Measure deconfinement and chiral symmetry restoration under
the conditions of maximum particle or energy density.

Before QCD
Density of hadron mass
states dN/dM increases
exponentially with mass.
dN


~ exp M 
 TH 
dM
TH ~ 21012 oK
Broniowski, et.al. 2004
Energy diverges as T --> TH
Rolf Hagedorn
German
Hadron bootstrap
model and limiting
temperature (1965)
Maximum achievable temperature?
“…a veil, obscuring our view of the very
beginning.” Steven Weinberg, The First Three
Minutes (1977)
QCD to the rescue!
Replace Hadrons
(messy and
numerous)
“In 1972 the early universe seemed
hopelessly opaque…conditions of
ultrahigh temperatures…produce a
theoretically intractable mess. But
asymptotic freedom renders
ultrahigh temperatures friendly…”
Frank Wilczek, Nobel Lecture
(RMP 05)
e/T4
 g*S
Thermal QCD
”QGP”
Hadron gas
by Quarks and
Gluons (simple
and few)
(Lattice)
Karsch, Redlich, Tawfik,
Eur.Phys.J.C29:549-556,2003
Nobel prize for Physics 2005
e+e- Annihilation
Nucleosynthesis
Mesons
freeze out
QCD Transition
Heavy quarks and
bosons freeze out
Thermal QCD -i.e. quarks and
gluons -- makes
the very early
universe
tractable; but
where is the
experimental
proof?
g*S
n Decoupling
“Before [QCD] we could not go back further than 200,000 years after the
Big Bang. Today…since QCD simplifies at high energy, we can extrapolate
to very early times when nucleons melted…to form a quark-gluon plasma.”
David Gross, Nobel Lecture (RMP 05)
Kolb & Turner, “The Early Universe”
Generating a deconfined state
Present understanding of
Quantum Chromodynamics (QCD)
• heating
• compression
 deconfined color matter !
Hadronic
Nuclear
Matter
Matter
Quark
Gluon
Plasma
(confined)!
deconfined
Expectations from Lattice QCD
e/T4 ~ # degrees of freedom
confined:
few d.o.f.
deconfined:
many d.o.f.
TC ≈ 173 MeV ≈ 21012 K ≈ 130,000T[Sun’s core]
eC  0.7 GeV/fm3
Suggested Reading
• October 2006 issue of Nature:
“Did the Big Bang Boil ? ” by F. Wilczek
• …the answer as far as the quarkhadron transition is concerned is
‘No’. QCD evolves smoothly with
temperature there is no
thermodynamic phase transition.
• Heavy Ion collisions at RHIC and
the LHC can produce fireballs with
a significant excess of baryons over
anti-baryons, or different effective
temperatures for quarks and gluons
– possibilities that did not occur in
the cosmic Big Bang. In those new
circumstances do true phase
transitions occur ?
A phase transition into what ?
• With the liquid-gas phase transition established (ground state
liquid drop nuclei transition to a hadron gas) the question was:
What comes next ? A weakly interacting plasma.
• Edward Shuryak (1971) : name it the Quark Gluon Plasma
Cabibo-Parisi, PLB59 (1975) G.Baym, NSAC-LRP (1983)
The phase diagram of QCD
Temperature
Early universe
critical point ?
quark-gluon plasma
Tc
colour
superconductor
hadron gas
nucleon gas
nuclei
CFL
r0
vacuum
baryon density
Neutron stars
Study all phases of a heavy ion collision
If the QGP was formed, it will only live for 10-22 s !!!!
BUT does matter come out of this phase the same way it went in ???
microexplosions
femtoexplosions
s
0.1 J
1 J
e
1017 J/m3
5 GeV/fm3 = 1036 J/m3
T
106 K
200 MeV = 1012 K
rate
1018 K/s
1035 K/s
Energy density of matter
high energy density:
e > 1011 J/m3
P > 1 Mbar
I > 3 X 1015W/cm2
Fields > 500 Tesla
QGP energy density
e > 1 GeV/fm3
i.e. > 1030 J/cm3
Step 1: Measuring a reference system
In order to prove that we form a phase of matter that
behaves different than the vacuum we need to
understand our results in pp collisions ?
Hadronization in QCD
(the factorization theorem)
Jet: A localized collection of hadrons
which come from a fragmenting parton
hadrons
c
a
Parton Distribution Functions
Hard-scattering cross-section
b
d
hadrons
Fragmentation Function
leading
particle
High pT (>~ 2.0 GeV/c) hadron production in pp collisions:
h
d pp
0
D
d

2
2
h/c

K
dx
dx
f
(
x
,
Q
)
f
(
x
,
Q
)
(
ab

cd
)

a
b a
a
b
b
2

dyd pT
dtˆ
zc
abcd
“Collinear factorization”
0 in pp: well described by NLO (& LO)
p+p->0 + X
Thermallyshaped Soft
Production
“Well Calibrated”
Hard
Scattering
• Ingredients (via KKP or Kretzer)
– pQCD
– Parton distribution functions
– Fragmentation functions
• ..or simply PYTHIA…
hep-ex/0305013 S.S. Adler et al.
pp at RHIC: Strangeness formation in QCD
nucl-ex/0607033
How strong are the NLO corrections
in LO calculations (PYTHIA) ?
• K.Eskola et al.
(NPA 713 (2003)):
Large NLO
corrections not
unreasonable at
RHIC energies.
Should be negligible
at LHC (5.5 or 14 TeV).
STAR
LHC
New NLO calculation based on STAR data
(AKK, hep-ph/0502188, Nucl.Phys.B734 (2006))
K0s
apparent Einc dependence of separated
quark contributions.
Mt scaling in pp
Breakdown of mT scaling in pp ?
mT slopes from PYTHIA 6.3
Gluon dominance at RHIC
PYTHIA: Di-quark structures in baryon production cause mt-shift
Recombination: 2 vs 3 quark structure causes mt shift
Collision Energy dependence of baryon/meson ratio
- baryon production in pp is simply not well understood
Ratio vs pT seems very energy dependent
(RHIC < < SPS or FNAL), LHC ?
Not described by fragmentation !
(PYTHIA ratios at RHIC and FNAL are equal)
Additional increase with system size in AA
Both effects (energy and system size
dependence) well described by recombination
Conclusions for RHIC pp data
• We are mapping out fragmentation and
hadronization in vacuum as a function of flavor.
• What we have learned:
– Strong NLO contribution to fragmentation even for light quarks at RHIC
energies
– Quark separation in fragmentation function very important. Significant nonvalence quarks contribution in particular to baryon production.
– Gluon dominance at RHIC energies measured through breakdown of mt-scaling
and baryon/meson ratio. Unexpected small effect on baryon/antibaryon ratio
– Is there a way to distinguish between fragmentation and recombination ? Does it
matter ?
• What will happen at the LHC ? What has happened
in AA collisions (hadronization in matter) ?
The future: unprecedented physics
reach at LHC (ALICE – pp)
(charged particle spectra)
enormous reach in multiplicity and transverse momentum.
Could this system behave collectively ??
Step 2: Proving the existence of a new phase of matter
Can we prove that we have a phase that
behaves different than elementary pp collisions ?
Three steps:
a.) prove that the phase is partonic
b.) prove that the phase is collective
c.) prove that the phase characteristics are different from the QCD
vacuum
Fate of jets in heavy ion collisions?
idea: p+p collisions @ same
sNN = 200 GeV as reference
p
p
?: what happens in Au+Au to jets
which pass through medium?
Prediction: scattered quarks
radiate energy (~ GeV/fm) in the
colored medium:
 “quenches” high pT particles
 “kills” jet partner on other side
?
Au+Au
Major discoveries in AuAu collisions
‘The Big Three’
(leading to the discovery of the sQGP
= the Perfect Quark Gluon Liquid
= AIP Science Story of 2005)
# I: The medium is dense and partonic
STAR, nucl-ex/0305015
pQCD + Shadowing + Cronin
energy
loss
pQCD + Shadowing + Cronin + Energy Loss
Deduced initial gluon density at t0 = 0.2 fm/c dNglue/dy ≈ 800-1200
e≈ 15 GeV/fm3, eloss = 15*cold nuclear matter
(compared to HERMES eA or RHIC dA)
(e.g. X.N. Wang nucl-th/0307036)
An important detail: the medium might not be totally opaque
There are specific differences to the flavor of the probe
Experiment: there are
baryon/meson differences
Theory: there are two types of e-loss:
radiative and collisional, plus
dead-cone effect for heavy quarks
Flavor dependencies map out the process of in-medium modification
# II: The medium behaves like a liquid
Strong collective flow:
elliptic and radial
expansion with
mass ordering
z
y
x
requires partonic hydrodynamics:
strong coupling,
small mean free path,
lots of interactions
NOT plasma-like more like a perfect liquid (near zero viscosity, d.o.f. ?)
# III: The medium consists of constituent quarks ?
baryons
mesons
Recombination vs. Fragmentation
(a different hadronization mechanism in medium than in vacuum ?)
Recombination at moderate PT
Parton pt shifts to higher
hadron pt.
Fragmentation at high PT:
Parton pt shifts to lower
hadron pT
Recomb.
fragmenting parton:
ph = z p, z<1
recombining partons:
p1+p2=ph
Frag.
plasma
liquid ?
gas
liquid
Hirano, Gyulassy (2006)
Consequences of a perfect liquid
• All “realistic” hydrodynamic calculations for RHIC
fluids to date have assumed zero viscosity
h= 0  “perfect fluid”


– But there is a (conjectured) quantum limit: h

4
( Entropy Density ) 
4
s
– Where do
“ordinary”
fluids sit wrt
this limit?
– RHIC “fluid” might
be at ~2-3 on this
scale (!)
T=1012 K
Description might require new dimensions
• Expanding our theoretical tools – the Maldacena conjecture
– AdS/CFT for calculating static and dynamic properties of
strongly-coupled gauge theories
• There is a string dual to AdS/CFT: 4 dim. SUSY Yang-Mills
• Determine viscosity and entropy density in RHIC by calculating it in a 10-dim black
hole calculation
MULTIPLICITY
Entropy  Black Hole Area
c
c
DISSIPATION
Viscosity  Graviton
Color Screening
Absorption
Explaining the Connection
1) Weakly Coupled
(classical) gravity in
Anti-deSitter Space (AdS)
2)
Maldacena’s
conjecture
3) Strongly
Coupled
(Conformal)
gauge Field
Theories
(CFT)
Suggested Reading
• November, 2005 issue of Scientific
American
“The Illusion of Gravity” by J. Maldacena
•
A test of this prediction comes from the
Relativistic Heavy Ion Collider (RHIC)
at BrookhavenNational Laboratory,
which has been colliding gold nuclei at
very high energies. A preliminary
analysis of these experiments indicates
the collisions are creating a fluid with
very low viscosity. Even though Son and
his co-workers studied a simplified
version of chromodynamics, they seem to
have come up with a property that is
shared by the real world. Does this mean
that RHIC is creating small fivedimensional black holes? It is really too
early to tell, both experimentally and
theoretically. (Even if so, there is nothing
to fear from these tiny black holes-they
evaporate almost as fast as they are
formed, and they "live" in five
dimensions, not in our own fourdimensional world.)
In the past six months: >50 preprints on AdS/CFT !
• “The stress tensor of a quark moving through N=4
thermal plasma”, J.J. Friess et al., hep-th/0607022
Our 4-d
world
String
theorist’s
5-d world
The stuff formerly
known as QGP
Jet modifications
from wake field
Heavy quark
moving
through
the
Energy loss medium
from string
drag
An explosion of new papers based
on string duality to AdS/CFT
• J.Friess et al., hep-th/0607022
Stress tensor of a quark moving through a N=4 thermal plasma
• J.Friess et al., hep-th/0605292
Dissipation from a heavy quark moving through a N=4 super YangMills plasma
• Liu, Rajagopal, Wiedemann, hep-th/0607062
An AdS/CFT calculation of screening in a hot wind
• Liu, Rajagopal, Wiedemann, hep-th/0605178
Calculating the jet quenching parameter from AdS/CFT
The perfect liquid, when does it vaporize ?
SU(3) gauge theory
(2+1) flavor QCD
Resummed perturbative calculations from :
Blaizot, Iancu, Rebhan, hep-ph/0303185
Lattice data on pressure and entropy density at high temperatures can be described
by re-summed perturbation theory. At high T deviation from SB limit only 10%
Comparison with re-summed perturbation theory, effective 3d theory and additional
lattice data on quark number susceptibility and the Debye mass suggest that
we have wQGP for T > 2 Tc
Quo Vadis ?
Many very important issues are left to investigate, e.g.
a.) need evidence for chiral symmetry restoration
b.) what are the initial conditions ?
c.) what is the hadronization mechanism in medium and in
vacuum ? How are hadronic masses generated ?
d.) is there a critical point at finite net baryon density and how
do the features of the phases change close to it ?
e.) is there a Color Glass Condensate at very low x ?
K*-(892)
Resonances are:
Luis Walter Alvarez
1968 Nobel Prize for
“ resonance particles ”
discovered 1960
• Excited state of a ground state particle.
• With higher mass but same quark content.
• Decay strongly  short life time
(~10-23 seconds = few fm/c ),
width = natural spread in energy:  = h/t.
Breit-Wigner shape
0
2
4
Number of events
10
8
6
Chirality: Why Resonances ?
640
680
720
760
800
840
880
920
Invariant mass (K0+) [MeV/c2]
minv 
E1  E2 )2  p1  p 2 )
2
K* from K-+p collision system
K  p  K*p
 K0 
• Broad states with finite  and t,
which can be formed by collisions between
the particles into which they decay.
Why Resonances?:
• Surrounding nuclear medium may change
resonance properties
• Chiral symmetry breaking:
Dropping mass -> width, branching ratio
Bubble chamber, Berkeley
M. Alston (L.W. Alvarez) et al., Phys. Rev. Lett. 6 (1961) 300.
Strange resonances in medium
Short life time [fm/c]
K* < *< (1520) < 
4 < 6 < 13 < 40
Rescattering vs.
Regeneration ?
Medium effects on resonance and their decay
Red: before chemical freeze out products before (inelastic) and after chemical
Blue: after chemical freeze out
freeze out (elastic).
Resonance Signal in Au+Au collisions
STAR
Preliminary
*0
K +
K(892)
K*0
Au+Au
minimum
biaspT  0.2
XX
*± +*±
GeV/c
|y|  0.5
(1520)
(1020)
STAR Preliminary
STAR Preliminary
Chiral Symmetry Restoration
Vacuum
At Tc: Chiral Restoration
Data: ALEPH Collaboration
R. Barate et al. Eur. Phys. J. C4 409 (1998)
Measure chiral partners
Near critical temperature Tc
(e.g. r and a1)
a1  r + 
Ralf Rapp (Texas A&M)
J.Phys. G31 (2005) S217S230
Lattice QCD predicts a critical point
(critical parameters at finite baryon density)
Critical Endpoint in Effective Models
Compilation by Stephanov
Do we need a color glass condensate to
explain experimental puzzle in HEP ?
Froissart Bound
The Gluon ‘blows up’
RHIC
The gluon ‘blows up’
Gluon saturation
Gluon density in hadrons
Motivation for the CGC
A time evolution for ‘matter’
Final phase diagram
The reach of RHIC
CGC at LHC
What are the initial conditions ?
– Mid – forward rapidity
correlations (hep-ph/0403271)
– Direct photons at forward
rapidities
– gg HBT (coherence of sea-quark
source?)
– Drell-Yan in forward region (hepph/040321)
– RpA, RAA of heavy mesons in
forward direction (hepph/0310358)
requires tracking, calorimetry and
PID over large h-range.
ln (1/x)
Color Glass Condensate (CGC):
gluon saturation at low Q2.
Measure
Onium physics – the complete program
– Melting of quarkonium states (Deconfinement TC)
Tdiss(Y’) < Tdiss((3S)) < Tdiss(J/Y)  Tdiss((2S)) <
Tdiss((1S))
Color screening of heavy flavor will tell us the
Initial temperature and its evolution with time !
The initial thermal conditions
Requirements for a complete onium program
• Full coverage high resolution forward calorimetry in order to
measure not only the J/y but also the cc and the Y States
Full coverage (|h|< 3) calorimetry and muon absorbers give us up to
1,000,000 cc, 10,000 Y(2s) and 10,000 Y(3s) per RHIC year. The J/y
alone is not sufficient !
There is plenty to do…
• ..and all of it is exciting
• ..and all of it is fundamental
• ..and all of it will benefit the understanding of
QCD, the standard model, and potentially new
physics
• ..and all of it will shed light on the evolution of
the universe
• ..and we might understand the generation of
mass, one of the most fundamental principles in
nature