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Relativistic Heavy Ion Physics:
the State of the Art
outline
 Science goals of the field
 Structure of nuclear matter and
theoretical tools we use
 Making super-dense matter in the
laboratory
the Relativistic Heavy Ion Collider
 experimental observables &
what have we learned already?
 Next steps...
Studying super-dense matter by
creating a little bang!
Structure of
atoms, nuclei,
and nucleons
At very high energy
shatter nucleons into
a cloud of quarks
and gluons
Expect a phase transition to a quark gluon plasma
Such matter existed just after the Big Bang
At high temperature/density
 Quarks no longer bound into nucleons
( qqq ) and mesons (qq )
Phase transition  quarks move
freely within the volume
 they become a plasma
* Such matter existed in the early universe
for a few microseconds after Big Bang
* Probably also in the core of neutron stars
Phase Transition
 we don’t really understand
how process of quark confinement works
how symmetries are broken by nature 
massive particles from ~ massless quarks
 transition affects evolution of early universe
latent heat & surface tension 
matter inhomogeneity in evolving universe?
more matter than antimatter today?
 equation of state  compression in stellar
explosions
Quantum ChromoDynamics
 Field theory
for strong interaction
among colored quarks
by exchange of gluons
 Works pretty well...
 Quantum Electrodynamics (QED)
for electromagnetic interactions
exchanged particles are photons
electrically uncharged
 QCD: exchanged gluons have “color”charge
 a curious property: they interact among
themselves
+
+…
This makes interactions difficult to calculate!
Transition temperature?
QCD “simplified”: a 3d grid of quark
positions & summing the interactions
predicts a phase transition:
e/T4
Karsch, Laermann, Peikert ‘99
T/Tc
Tc ~ 170 ± 10 MeV (1012 °K)
e ~ 3 GeV/fm3
So, we need to create a little
bang in the lab!
Use accelerators to reach highest energy
vBEAM = 0.99995 x speed of light at RHIC
center of mass energy s = 200 GeV/nucleon
SPS (at CERN) has s  18 GeV/nucleon
AGS (at BNL) s  5 GeV/nucleon
Use heaviest beams possible
maximum volume of plasma
~ 10,000 quarks & gluon in fireball
Experimental method
Collide two nuclei
Look at region between the two
nuclei for T/density maximum
RHIC is first dedicated heavy ion collider
10 times the energy previously available!
RHIC at Brookhaven
National Laboratory
Relativistic Heavy Ion Collider
started operations in summer 2000
4 complementary experiments
STAR
What do we need to know
about the plasma?
 Temperature
early in the collision, just after nuclei
collide
 Density
also early in the collision, when it is at its
maximum
 Are the quarks really free or still
confined?
 Properties of the quark gluon plasma
equation of state (energy vs. pressure)
how is energy transported in the plasma?
In Heavy Ion Collisions
When nuclei collide at near the speed of
light, a cascade of quark & gluon
scattering results….
104 gluons, q, q’s
Is energy density high enough?
PRL87, 052301 (2001)
Colliding system expands:
Energy  to
beam direction
R2
2c0
e Bj
1
1

R 2 2c 0
 dET
 2
 dy



per unit
velocity || to beam
 e  4.6 GeV/fm3
YES - well above predicted transition!
50% higher than seen before
Density: a first look
Central Au+Au
collisions
(~ longitudinal velocity)
Adding all particles under the curve,
find ~ 5000 charged particles
These all started in a volume ~ that of a nucleus!
Observables II
Density - use a unique probe
schematic view of jet production
Probe: Jets from
scattered quarks
hadrons
leading
particle
Observed via fast
leading particles or
azimuthal correlations
between the leading
particles
hadrons
q
q
leading particle
But, before they create jets, the scattered
quarks radiate energy (~ GeV/fm) in the
colored medium
 decreases their momentum
 fewer high momentum particles  beam
 “jet quenching”
See talk by X.N. Wang
Deficit observed in central
collisions
Yieldcentral /  Ncoll  central
Yieldpp
charged
0
Charged deficit
seen by both
STAR & PHENIX
See talk by F. Messer
transverse momentum (GeV/c)
Observables III
Confinement
 J/Y (cc bound state)
 produced early, traverses the medium
 if medium is deconfined (i.e. colored)
other quarks “get in the way”
J/Y screened by QGP
binding dissolves  2 D mesons
u, d, s
u, d, s
c
c
See talks of D. Kharzeev & J. Nagle
J/Y suppression observed
at CERN
NA50
J/Y
yield
Fewer J/Y in Pb+Pb than expected!
But other processes affect J/Y too
so interpretation is still debated...
RHIC data being analyzed now !
Observables IV: Properties
elliptic flow “barometer”
Origin: spatial anisotropy of the system when created
followed by multiple scattering of particles in evolving system
spatial anisotropy  momentum anisotropy
v2: 2nd harmonic
Fourier coefficient in
azimuthal distribution
of particles with
respect to the
reaction plane
Almond shape
overlap region in
coordinate space
y 2  x 2 
e 2
2
y  x 
v2  cos2
  atan
py
px
Large v2: the matter
can be modeled hydrodynamics
v2 = 6%: larger than at CERN or AGS!
Hydro. Calculations
Huovinen,
P. Kolb and
U. Heinz
STAR
PRL 86 (2001) 402
pressure buildup  explosion
pressure generated early!
 early equilibration !?
first hydrodynamic behavior seen
Observables V
Temperature
Look for “thermal” radiation
processes producing it:
e-, m-
q
g*
Thermal dilepton
radiation:
e+, m+
q
Thermal photon
radiation:
q, q
g
g
Rate, energy of the radiated particles
determined by temperature
NB: g, e, m interact electromagnetically only
 they exit the collision without further
interaction
See talk of D. Kharzeev
Temperature achieved?
 At CERN, photon
and lepton spectra
consistent with
T ~ 200 MeV
NA50
WA98
m m pairs
photons
 At RHIC we don’t know yet
 But it should be higher since the energy
density is larger
The state of the art
(and the outlook…)
 unprecedented energy density at RHIC!
high density, probably high temperature
very explosive collisions  matter has a
stiff equation of state
 new features: hints of quark gluon plasma?
large elliptic flow, suppression of high pT,
J/Y suppression at CERN?
but we aren’t sure yet…
 To rule out conventional explanations
extend reach of Au+Au data
 compare p+p, p+Au to check effect of
cold nuclei on observables
 study volume & energy dependence

Mysteries...
 How come
hydrodynamics
does so well on
elliptic flow and
momentum spectra
of mesons &
nucleons emitted
… but FAILS to
explain correlations
between meson PAIRS?
not explosive enough!
pT (GeV)
 If jets from light quarks are quenched,
shouldn’t charmed quarks be suppressed too?
See talk of J. Nagle
Compare spectra to p+p collisions
Peripheral collisions (60-80% of sgeom):
~ p-p scaled by <N bin coll> = 20  6
central (0-10%):
shape different (more exponential)
below scaled p-p!
(<N bin coll> = 905  96)
Did something new happen?
 Study collision dynamics
Do the particles
equilibrate?
Collective behavior
i.e. pressure and
expansion?
 Probe the early (hot) phase
Particles created early
in predictable quantity
interact differently with
QGP and normal matter
fast quarks, bound
cc pairs, s quarks, ...
+ thermal radiation!
matter box
vacuum
QGP
Thermal Properties
measuring the thermal history
g, g* e+e-, m+m , K, p, n, , L, D, X, W, d,
PCM & clust. hadronization
Real and virtual
photons from quark Hadrons
NFD reflect thermal
properties when inelastic
scattering is most
NFD & hadronic TM
collisions
stop (chemical
sensitive to the early
string & hadronic TM
freeze-out).
stages. (Run II
PCM & hadronic TM
measurement)
Hydrodynamic flow is sensitive to the
entire thermal history, in particular the
early high pressure stages.
CYM & LGT