Modern Nuclear Physics with STAR @ RHIC:
Recreating the Creation of the Universe
Rene Bellwied
Wayne State University
(bellwied@physics.wayne.edu)
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Lecture 1: Why and How ?
Lecture 2: Bulk plasma matter ?
(soft particle production)
Lecture 3: Probing the plasma
(via hard probes)
0.) global observables
A.) particle production
B.) particle spectra
C.) particle flow
D.) particle correlations
Collective Radial Expansion
From fits to p, K, p spectra:


Slightly model dependent
here:
Blastwave model
< r >
 increases continuously
Tth
 saturates around AGS energy
Strong collective radial
expansion at RHIC
 high pressure
 high rescattering rate
 Thermalization likely
Elliptic (anisotropic) Flow for a mid-peripheral collision
Out-of-plane
Flow
– a strong indicator of collectivity
Y
dpt dy dφ dpt dy 2π
2v 2 cos ( 2φ)  ...)In-plane
Reaction d N  d N 1 ( 1 2v1 cos (φ ) Flow
3
2
X
plane
Directed flow
Dashed lines: hard
sphere radii of nuclei
Elliptic flow
Re-interactions  FLOW
Re-interactions among what? Hadrons, partons or both?
In other words, what equation of state?
v2 measurements
(Gyulassy’s proof of parton collectivity)
System deformation in HBT
Y
Time
X


y2  x2
y2  x2
Time
Final state eccentricity
from
 v2
 HBT with respect to
reaction plane
Lifetime and centrality dependence
from (1520) / and K(892)/K
G. Torrieri and J. Rafelski, Phys. Lett. B509 (2001) 239
Life time:
K(892) = 4 fm/c
(1520) = 13 fm/c
Model
Blast wave
includes:
fit of p,K,p (Tkin +)  Tchem
• 
Temperature
~ 6 fm/cat chemical freeze-out
• Based
Lifetime
between t
chemical
thermal
freeze-out
on entropy:
~ (Tchand
/Tkin
– 1) R/
s
• By comparing two particle ratios (no regeneration)
 does not change much with centrality
results
between
: reduction is compensated by
because
slight T
T=
160 MeV
=> velocity
> 4 fm/c
limit !!!)
slower
expansion
 in(lower
peripheral

= 0 fm/c => T= 110-130 MeV
collisions.
preliminary
(1520)/ = 0.034  0.011  0.013
K*/K- = 0.20  0.03 at 0-10% most central Au+Au
More resonance measurements are needed
to verify the model and lifetimes
Time scales according to STAR data
initial state
hadronic phase
and freeze-out
QGP and
hydrodynamic expansion
pre-equilibrium
hadronization
Balance
CYM & LGTfunction (require flow)
Resonance survival
PCM & clust. hadronization
NFD
dN/dt
Rout, Rside
Rlong (and HBT wrt reaction
plane)
NFD & hadronic TM
string & hadronic TM
PCM & hadronic TM
1 fm/c
5 fm/c
Chemical freeze out
10 fm/c
20 fm/c time
Kinetic freeze out
Summary: global observables
Initial energy density high enough to produce
a QGP

  10 GeV/fm3
(model dependent)

High gluon density
dN/dy ~ 8001200


Proof for high density matter but not for QGP
Summary of particle identified observables
Statistical thermal models appear to work well at SPS and RHIC
 Chemical freeze-out is close to TC
 Hadrons appear to be born
into equilibrium at RHIC (SPS)
 Shows that what we observe is
consistent with thermalization
 Thermal freeze-out is common
for all particles if radial flow
is taken into account.
T and T are correlated
 Fact that you derive T,T is
no direct proof but it is consistent with thermalization
Conclusion
There is no “
“ in bulk matter properties
 However:
 So far all pieces point
indeed to QGP formation
- collective flow
elliptic & radial
- thermal behavior
- high energy density
- strange particle production enhancement

Lecture 3
Probing the plasma ?
Are we sensitive to its
properties ?
Can we find relevant probes ?
Distinction between probe and medium

We are producing ‘soft’ and ‘hard’ matter. An
arbitrary distinction is coming from the
applicability of pQCD which is generally set
to pT > 2 GeV/c (hard). Below pT = 2GeV/c
we expect thermal bulk matter production.
Medium: The bulk of the particles; dominantly
soft production and possibly exhibiting some
phase.
 Probe: Particles whose production is calculable,
measurable, and thermally incompatible with
(distinct from) the medium (hard production)

The Probes Gallery and its signatures
Jet ‘Quenching’
charm/bottom dynamics
J/Y & U
direct photons
CONTROL
What is happening ?
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Two partons interact strongly (e.g. gluon fusion, gluon
annihilation, quark scattering).
The two resulting partons fragment into two jets.
This reaction is governed by two calculable or
normalizable functions:
 The initial parton distribution function
 The final parton fragmentation function
Theorists have speculated that both of these functions
could exhibit medium modifications in dense partonic
matter
Modification of PDF: Color Glass Condensate (CGC)
Modification of FF: Jet Quenching
What is a Jet?

Jets arise from hard
partonic collisions
 High pt partons
fragment into a
collimated spray
of particles.
z= pt/p1
Jt
parton
hadrons
parton
In general, the cone is not
bisected by the thrust axis.
Why is Jet Reconstruction Important?

Observables from fully
reconstructed jets compare
directly with pQCD theory
 Reconstructed Et
approximates parton p1
 Reduces fragmentation
function ambiguities
z= pt/p1
Jt
parton
hadrons
Full jet reconstruction is very difficult in
AA because of large thermal background.
parton Measure leading particle instead of full jet.
Each jet has a leading particle that carries
on average around 30% of the full jet energy.
The reference: Two jet events in the
STAR EMC in a p+p collision
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Neutral energy (which
includes p0 decay photons) is
measured with the STAR
Electromagnetic Calorimeter
In Au+Au collisions these
jets only rise above the
background for jet energies
exceeding ~ 20 GeV.
Besides two jet events also
three jet events are seen in
elementary collisions. In that
case the 3rd jet is a gluon jet
How do I calculate the pp yield ?

pQCD
 Initial parton momentum fraction probabilities from
parton distribution functions (PDF)
parametrization: LO Glueck-Reya-Vogt (GRV98)
 Momentum fraction carried away by leading hadron
from fragmentation functions (FF)
parametrization: LO Binnewies-Kniehl-Kramer (BKK)
 Constant K-factor and parton kT broadening function
to account for NLO corrections
Parton distribution functions
(hep-ex/0305109)
Fragmentation
functions for
different baryons
Bourelly & Soffer
hep-ph/0305070
Does pQCD work in pp at RHIC ?
p+p->p0 + X
Thermallyshaped Soft
Production
“Well Calibrated”
Hard

pQCD approach for Ed3s/dp3


Point-like scattering process a+b→c+d
(via vector gluon exchange) (Berman,
Bjorken, Kogut 1971)
2
 ds/dt ~ 1/s
Ed3s/dp3 ~ pT-4
Constituent Interchange Model and
quark-fusion model
 add other subprocesses (quarkmeson,quark-diquark scattering)
 n = 8 for pions, n = 12 for baryons
Scattering
hep-ex/0305013 S.S. Adler et al.
The idea of jet quenching (Wang, Gyulassy)

If the initial phase is a QGP then a jet
traversing the dense matter and
fragmenting should experience a
different (larger) energy loss than in pp
or cold nuclear matter
Specifically: a jet should lose energy in
the medium via gluon bremsstrahlung
q/g jets as probe of hot medium
schematic view of jet production
Jets from hard scattered
quarks observed via fast
leading particles or
azimuthal correlations
between the leading
particles
hadrons
leading
particle
q
q
hadrons
leading particle
However, before they create jets, the scattered quarks
radiate energy (~ GeV/fm) in the colored medium
decreases their momentum (fewer high pT particles)
“kills” jet partner on other side
Modification of fragmentation functions
(e.g.hep-ph/0005044)


Induced Gluon Radiation
~collinear gluons in cone
“Softened” fragmentation
Gyulassy et al., nucl-th/0302077
in jet
ch
n
z
: increases
in jet
: decreases
Partonic vs. Hadronic Mechanisms
Fragmentation:
phadron
z
p parton
Rcone




Hottest Questions:
Fragment inside/outside of
medium?
Partonic vs. hadronic model
of final State interaction?
A Better Question:
To what degree do partonic
and hadronic interactions
contribute to quenching?
Discussion only recently
started!
High pT hadrons in Au+Au
The Suppression Factors
Nuclear modificati on factor :
2
AA
d N dpT dy
RAA 
2 NN
TAA d s
dpT dy
Geometric modificati on factor :
R
geo
AA
Peripheral
bin
d N
dpT dy N
 2 Peripheral
Central
d N
dpT dy N bin
2
Central
Suppression of inclusive charged hadrons
Central normalized to NN (130 GeV)
Central/peripheral (200 GeV)
Preliminary
• Evidence for high pT hadron suppression in central collisions
• Suppression factor ~constant for 6<pT<12 GeV/c
Nuclear modifications to hard scattering
Large
enhancement
effect at SPS
and ISR
Suppression
at RHIC
Are there ‘normal’ nuclear effects ?

Nuclear Shadowing
 Nuclear modifications to the parton distribution
function in cold nuclear matter, measured and
parametrized in the ‘shadowing function’ (EKS98)

Cronin effect
 Multiple initial state scatterings of partons in cold
nuclei lead to high pt particle enhancement compared
to pp reactions and to kT broadening. Parametrized
through kT broadening functions, which also includes
‘random walk’ elastic scattering corrections.
Past pA measurements (cold nuclear matter)
P.B. Straub et al., PRL 68 (1992)
Fermilab experiments measuring
R (W / Be) for identified particles
at s of 27.4 and 51.3 GeV.
Interpretation:
Cronin effect drives all R(AA)
above unity. Then at high pt all
R(AA) approach unity which is
the QCD limit.
Preliminary RHIC dA data:
Interesting observation:
Cronin
effect seems
to be mass
20%
difference
between
and / or quark content dependent.
Pions
and Protons
(e.g. increased probability of
gluon rescattering in nuclear
No
difference
matter
relative tobetween
quarks is
attributed
to stronger
K- effect)
Proton
and
Lambdas
Gluon interaction in cold and hot matter
Gluon interactions occur already in cold nuclear matter but
the effects are different (A. Krzywicki et al., PLB 85,1979)
In cold nuclear matter the triple-gluon
coupling favors multiple gluon scattering,
the fraction of gluon jets is enhanced at
large pt, i.e.softening of gluon
fragmentation function. Measurable by
comparing K- (more gluon contribution) to
K+ (more quark contribution) leading
particles -> Gluon Filter
In hot nuclear matter the gluon interaction
via gluon bremsstrahlung is enhanced even
further (estimate by X.N.Wang by
comparing RHIC to HERA data: factor 15
from 0.5 GeV/fm to 7.3 GeV/fm)
Does the gluon filter survive ? ( vs. )
Gluon bremsstrahlung
E  gluon density
33
Gyulassy-Levin-Vitev predictions
Includes Cronin, Shadowing, and
Quenching. Free parameter: initial
gluon density.
At SPS Cronin dominates but is still
reduced by a factor 2 due to moderate
jet quenching.
Initial gluon density ~ 200
At RHIC jet quenching dominates, but
as a f(pt) Cronin and quenching yield
an almost constant RAA.
Initial gluon density ~ 1000
At LHC hadronic fragments from
energetic jets compensate the
increasing jet quenching.
Initial gluon density ~ 3000
Azimuthal high pt two particle
correlations in Au+Au
Au+Au peripheral
?
Au+Au central
Phys Rev Lett 90, 082302
Suppression of back-to-back correlations in central Au+Au
Details on away-side suppression
Peripheral Au + Au

C2 (Au  Au)  C2 ( p  p)  A * (1 2v 22 cos(2 ))
• Near-side well described
• Away-side suppression
in central collisions
STAR Preliminary
Preliminary
near side
Central Au + Au
away side
Away side hadrons are suppressed!
Is suppression due to initial or final state ?
Initial
state?
Final state?
strong modification of
Au wavefunction (gluon
saturation)
partonic energy loss in
dense medium
generated in collision
Ultimate test: dA collisions
Two ideas: initial or final state effect ?
Initial state effect: gluon saturation
Formation of new state of matter: Color Glass Condensate because
of gluon saturation in Lorentz contracted nucleus at low x. Depends
on parton packing factor. Structure functions will be modified. If
gluon saturation is valid then pQCD is not valid !
Final state effect: jet quenching in medium
Energy loss in medium due to gluon radiation (QGP signature).
Fragmentation functions will be modified.
Details of Color Glass Condensate model
At RHIC: Qs2 = 2 GeV2, pt=4 GeV/c
Summary: initial & final state effects
The ultimate control experiment: d+Au
Nucleusnucleus
collision



Proton/deuteron
nucleus
collision
Collisions of small with large nuclei were always foreseen
as necessary to quantify cold nuclear matter effects.
Small + Large distinguishes all initial and final state
effects.
Predictions: if final state effect, then dA shows no jet
suppression or back-to-back disappearance. If initial state
effect, then the jet suppression effects will still be strong.
Nuclear suppression factors in dA
Cronin Effect:
Multiple Collisions
broaden high PT
spectrum

Striking difference of d+Au and Au+Au results.
Enhancement instead of suppression due to Cronin effect
in cold nuclear matter ?
Centrality Dependence
Au + Au Experiment
Final Data
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
d + Au Control Experiment
Preliminary Data
Dramatically different and opposite centrality evolution
of Au+Au experiment from d+Au control.
Jet Suppression is clearly a final state effect.
dA azimuthal distributions
pedestal and flow subtracted
Suppression of the back-to-back correlation
in central Au+Au is a final-state effect
Is the CGC dead?
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NO. The arguments for gluon saturation are very solid
theoretically.
The results from mid-rapidity and high pT have certainly
been proven to utilize a high enough x range that the
gluon saturation effects are not significant.
Nonetheless when probed at sufficiently low-x the Au
wave function should exhibit saturation (fewer
scattering centers).
Collisions involving a low-x parton from Au and a highx parton from d, will be highly forward focussed:

Prediction RAA<1 at high forward rapidity.
RdAu at different rapidities
Particle Identified Jets (leading hadrons):
Strangeness pushing the boundaries
preliminary
preliminary
Mass dependence of the transverse expansion, which is
well described by thermal and hydrodynamics models
Strange jet-like particles
The v2 saturation and the
the decrease in RAA appear
to be loosely correlated for
both KS and Λ.
What physics is behind the
pt scales of the saturation
in v2 and the suppression
in RAA.
R(CP) for all strange hadrons!
 Two groups (2<pt<6GeV/c)
- K0s, K, K*,   mesons
- , X, W
 baryons
 dependence on number of
valence quarks
 limited to pt<6GeV/c ?
 hadron production from
quark coalescence ?
Elliptic flow moment (v2) for strange hadrons
scaled with n(quarks)
scaling seems to work
(not for pions...)
 Parton recombination?

STAR Preliminary
Baryon excess at mid pt in central collisions
The ‘intermediate’ pt region
pT independence of
pbar/p ratio.
 p/p ratio increases
with pT to ~ 1 at pT
~ 3-4are
GeV/c
What
thein
central collisions.
mechanism(s)
at
 Suppression
factors of p, pT?
intermediate
different to that of
p, K0s in the
intermediate pT
region.

Soft
0
2-3 GeV/c
Fragmentation
and quenching
of jets
6-7 GeV/c ?
pT
Current theoretical models



Soft + Quench Model M. Gyulassy et. al., Phys. Rev. Lett. 86 (2001) 2537
 Two component model, soft production (hydro) at low pT,
quenching of pQCD jets via gluon radiation at higher pT.
 Baryon junctions incorporated to explain large baryon yield
at intermediate pT.
Recombination R. J. Fries et. al., Phys. Rev. C 68 (2003) 044902
 Model assumes the recombination of two and three low pT
partons to form hadrons from an exponential parton pT
spectrum. High pT spectrum described by fragmentation once
parton pT spectrum can be described by a power law.
 Requires a high phase space density of partons for method
to work.
Coalescence V. Greco et. al., Phys. Rev. C 68 (2003) 034904
 Same as the recombination picture with the added assumption
that thermal ‘QGP’ partons can coalesce with co-moving ‘pQCD’
partons from a mini-jet.
/K0s - comparison with models
• S+Q : magnitude
turnover
centrality
yes
yes
yes
• Reco : magnitude yes
turnover yes
low pT
no
• Coal : magnitude no
turnover
no
low pT
yes
Require centrality dependent prediction
from Recombination and Coalescence
models.
S+Q :: 200
GeV
data
- private
Reco
Phys.
Rev.
C68
044902communication
(2003)
S+Q
GeV
data
- Phys.
Rev.
C65, 041902
Coal ::130
Phys.
Rev.
C68
034904
(2003)
What is our present conclusion ?
The interpretation of bulk properties in heavy ion systems is complex. We have
indications of unusual behavior in rare, fast decoupling, and high momentum
probes. Our system behaves like matter, not a collection of elementary particles,
and we have the tools to study it.


We could declare discovery of the QGP but we have more things to study and
we don’t have the ‘smoking gun’ yet. Further exploration will take a few years,
but the first steps were very exciting and very successful.
Have we found the
Quark Gluon Plasma at RHIC?
We now know that Au+Au collisions generate a medium that
 is dense (pQCD theory: many times cold nuclear matter density)
 is dissipative
 exhibits strong collective behavior
 shows hints of thermalization and collectivity at very early stage
This represents significant progress in our understanding of
strongly interacting matter
We have yet to show that:
a.) dissipation and collective behavior both occur at the partonic stage
b.) the system is deconfined and thermalized
c.) a transition occurs: can we turn the effects off ?
Not yet, there is still work to do
Suggestions for further reading

QCD: The Modern Theory of the Strong Interaction
F. Wilczek, Ann. Rev. Nucl. Part. Sci. 32 (1982) 177

Symmetry breaking and quark confinement
Concepts of Particle Physics Vol I and II
K. Gottfried and V.F. Weisskopf, Oxford University Press (1984)

The Transition from Hadron Matter to Quark-Gluon Plasma
H. Satz, Ann. Rev. Nucl. Part. Sci. 35 (1985) 245

Introduction to High-Energy Heavy-Ion Collisions
C.-Y. Wong, World Scientific (1994)

The Search for the Quark-Gluon Plasma
J. Harris and B. Muller, Ann. Rev. Nucl. Part. Sci. 46 (1996) 71