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The Quark Gluon Plasma
what is it and why should it exist ?
The first second of the universe
time
AP
NP
HEP
from D.J. Schwarz, astro-ph/0303574
History of the early universe
The tools need to study the early history of the universe:
- Accelerator based high energy and nuclear experiments (LHC, RHIC)
- Observational cosmology (LSST, HAWC, etc.)
Going back in time…
Age
0
Energy
1019 GeV
Matter in universe
grand unified theory of all forces
10-35 s
1014 GeV
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
3rd phase transition
RHIC, LHC & FAIR
0.1 MeV
nuclei
FRIB & FAIR
(strong:hadrons + electro:g + weak: l,n)
3 min.
6*105 years
0.3 eV
Now
3*10-4 eV = 3 K
(13.7 billion years)
atoms
Analogies and differences between QED and QCD
to study structure of an atom…
electron
…separate constituents
nucleus
F ~ 1/r2
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)
F~r
The main features of Quantum Chromodynamics
(QCD)
(Nobel Prize 2004)

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 quasifree.

(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
Theoretical and computational QCD
in vacuum and in medium
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.
B.Mueller (2004):
New Discoveries at RHIC
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 evolution of luminous matter
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)
QCD phase transition (I):
chiral symmetry breaking via
dynamical quarks. Mechanism
to generate constituent quark
masses (but does not explain
hadronization)
QCD phase transition (II):
Confinement to hadrons.
Mechanism to generate
hadron properties (but does
not explain hadron masses)
What can we do in the laboratory ?
a.) Re-create the conditions as close as possible to the
Big Bang, i.e. a condition of maximum density and
minimum volume in an expanding macroscopic system.
Is statistical thermodynamics applicable ?
b.) Measure a phase transition, characterize the new
phase, measure the de-excitation of the new phase into
‘ordinary’ matter – ‘do we come out the way went in ?’
(degrees of freedom, stable or metastable matter,
homogeneity)
c.) Learn about hadronization (how do particles acquire
mass) – complementary to the Higgs search but with
the same goal. The relevant theory is Quantum Chromo
Dynamics
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
/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]
C  0.7 GeV/fm3
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
Relativistic Heavy Ion Collider (RHIC)
PHOBOS
PHENIX
1 mile
Au+Au @ sRHIC
NN=200 GeV
BRAHMS
STAR
v = 0.99995c
AGS
TANDEMS
Study all phases of a heavy ion collision
If the QGP was formed, it will only live for 10-21 s !!!!
BUT does matter come out of this phase the same way it went in ???
Study all phases of a heavy ion collision
If the QGP was formed, it will only live for 10-21 s !!!!
BUT does matter come out of this phase the same way it went in ???
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 (state variables)
are different from the QCD vacuum
Proof (a) for partonic medium creation
Shooting a high momentum particle through a
dense medium
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
RAA and high-pT suppression
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
 ≈ 15 GeV/fm3, eloss = 15*cold nuclear matter (compared to HERMES eA)
(e.g. X.N. Wang nucl-th/0307036)
SYSTEM NEEDS TO BE PARTONIC
Mid-central
collision
Reaction
plane
Out-of-plane
Flow
Proof (b): is the matter behaving collective ?
elliptic (anisotropic) flow
Flow
Y
In-plane
X
Directed flow
Elliptic flow
Dashed lines: hard
sphere radii of nuclei
Y
Time
X
Proof (c): new phase leads to new matter production mechanism
The medium consists of constituent quarks ?
Massive quasiparticles instead of current quarks ?
baryons
mesons
Elliptic flow exists and its magnitude is
described by ideal hydrodynamics !
Strong collective flow:
elliptic expansion with
mass ordering
z
y
x
Hydrodynamics:
strong coupling,
small mean free path:
many interactions
NOT plasma-like
system behaves liquid-like
A surprise: ideal liquid behavior
First time in heavy-ion collisions we created a system which is in
quantitative agreement with ideal hydrodynamic model. The new
phase behaves like an ideal liquid rather than a plasma.
Not anticipated. In stark contrast to pQCD.
How strong is the coupling ?
Simple pQCD processes do not generate sufficient interaction strength.
Navier-Stokes type calculation of viscosity yield a near perfect liquid
Viscous force ~ 0. We have made a sQGP not the anticipated wQGP.
?
Experimental verification at RHIC
Lacey et al., PRL 98 (2007) 092301
RB, J.Phys.G35:044504 (2008)
The quantum limit has been reached at RHIC and has
been independently verified in several measurements
of collective effects
Lessons from
RHIC:
The Quark
Soup
AIP Science
Story of 2006
Hirano, Gyulassy (2006)
The future is bright
A three prong approach:
lower energy
better facility
higher energy
FAIR:
Facility for
Antiproton &
Ion
Reseach
RHIC-II:
RHIC upgrade
with higher
luminosity and
upgraded detectors
LHC:
Large Hadron Collider
with ALICE, CMS,
ATLAS
Measuring pp/AA in all LHC experiments

ALICE
 Dedicated & most versatile heavy ion
detector
 Important for particle identified
fragmentation measurements in pp

CMS/ATLAS
 Dedicated & most versatile p+p
detectors
 Important for calorimeter based jet
measurements in A+A.