Transcript Using Datasets

T. Hallman SC MTG Jan 2005
Evidence for the Production of the Quark-Gluon Plasma at RHIC
Tim Hallman
Scientific Council Meeting
Dubna, Russia
January 20-21, 2005
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T. Hallman SC MTG Jan 2005
A Definition of the Quark-Gluon Plasma
QGP  a (locally) thermally equilibrated state of matter in
which quarks and gluons are deconfined from hadrons,
so that color degrees of freedom become manifest over
nuclear, rather than merely nucleonic, volumes.
Not required:

non-interacting quarks and gluons

1st- or 2nd-order phase transition

evidence of chiral symmetry restoration
This definition is consistent within the community and over time
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T. Hallman SC MTG Jan 2005
Elliptic Flow at RHIC
Anisotropic (Elliptic) Transverse Flow
•
The overlap region in peripheral collisions is not
symmetric in coordinate space
– Almond shaped overlap region
• Easier for particles to emerge in the
direction of x-z plane
• Larger area shines to the side
– Spatial anisotropy  Momentum anisotropy
• Interactions among constituents generates
a pressure gradient which transforms the initial
spatial anisotropy into the observed momentum
anisotropy
•
Perform a Fourier decomposition of the
momentum space particle distributions in the x-y
plane
• v2 is the 2nd harmonic Fourier coefficient of the
distribution of particles with respect to the
reaction plane
Peripheral
Collisions
z
y
x
Anisotropic Flow
v2  cos 2   atan
py
px
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T. Hallman SC MTG Jan 2005
Soft Sector: Evidence for Thermalization and EOS with Soft Point?
Hydro calculations: Kolb, Heinz and Huovinen
 Systematic m-dependence of v2(pT) suggests common transverse vel. field
 mT spectra and v2 systematics for mid-central collisions at low pT are well
(~20-30% level) described by hydro expansion of ideal relativistic fluid
 Hydro success suggests early thermalization, very short mean free path
 Best agreement with v2 and spectra for therm < 1 fm/c and soft (mixed-phasedominated) EOS ~ consistent with LQCD expectations for QGP  hadron
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T. Hallman SC MTG Jan 2005
How Unique & Robust is Hydro Account in Detail?
 Are we sure that observed v2 doesn’t result
alternatively from harder EOS (no transition) and
late thermalization?
 How does sensitivity to EOS in hydro calcs.
compare quantitatively to sensitivity to other
unknown features: e.g., freezeout treatment
(compare figures at right), thermaliz’n time,
longitudinal boost non-invariance, viscosity?
 What has to be changed to understand HBT
(below), and what effect will that change have on
soft EOS conclusion?
Sharp freezeout
 dip
P. Kolb, J. Sollfrank,
and U. Heinz, Phys.
Rev. C. C62 054909
(2000).
Hydro+RQMD
 no dip?
Hydro vs. STAR HBT Rout/Rside
Teaney,
Lauret &
Shuryak
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T. Hallman SC MTG Jan 2005
Self-Analyzing (High pT) Probes of the Matter at RHIC
Nuclear
Modification
Factor:
d 2 N AA / dpT d
RAA ( pT ) 
TAAd 2 NN / dpT d
nucleon-nucleon
cross section
<Nbinary>/inelp+p
AA
leading
particle
suppressed
hadrons
q
q
?
If R = 1 here, nothing new
going on
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T. Hallman SC MTG Jan 2005
Hard Sector: Evidence for Parton Energy Loss in
High Density Matter
PHENIX
 Inclusive hadron and away-side correlation suppression in central Au+Au,
but not in d+Au, clearly establish jet
quenching as final-state phenomenon,
indicating very strong interactions of
hard-scattered partons or their
fragments with dense, dissipative
medium produced in central Au+Au.
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T. Hallman SC MTG Jan 2005
Questions for Parton Energy Loss Models
 pQCD parton energy loss fits to observed
central suppression  dNgluon/dy ~ 1000 at
start of rapid expansion, i.e., ~50 times cold
nuclear matter gluon density.
 ~pT-independence of measured RCP 
unlikely that hadron absorption dominates jet
quenching.
How sensitive is this quantitative conclusion to:
assumptions of factorization in-medium and
vacuum fragmentation following degradation;
treatments of expansion and initial-state cold
energy loss preceding hard collision?
 Can pQCD models account for orientationdependence of di-hadron correlation? Should be
sensitive to both path length and matter expansion
rate variation with (R).
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Soft Sector: Hadron Yield Ratios
T. Hallman SC MTG Jan 2005
STAR
PHENIX
Strangeness Enhancement
Resonances
 pT-integrated yield ratios in central Au+Au collisions consistent
with Grand Canonical stat. distribution @ Tch = (160 ± 10) MeV,
B  25 MeV, across u, d and s sectors.
 Inferred Tch consistent with Tcrit (LQCD)  T0 >Tcrit .
 Does result point to thermodynamic and chemical equilibration,
and not just phase-space dominance?
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T. Hallman SC MTG Jan 2005
Intermediate
pT: Hints of
Relevant Degrees of
Freedom
 For 1.5 < pT <6 GeV/c, see
clear meson vs. baryon
(rather than mass-dependent)
differences in central-to-midcentral yields and v2.
 v2/nq vs. pT /nq suggestive of
constituent-quark scaling. If
better established exp’tally,
would give direct evidence of
degrees of freedom relevant
at hadronization, and suggest
collective flow @ constituent
quark level.
 N.B. Constituent quarks 
partons! Constituent quark
flow does not prove QGP
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Questions for Coalescence Models
T. Hallman SC MTG Jan 2005
Duke-model
recomb. calcs.
Duke-model
recomb. calcs.
 Can one account simultaneously for spectra, v2 and di-hadron 
correlations at intermediate pT with mixture of quark recombination
and fragmentation contributions? Do observed jet-like near-side
correlations arise from small vacuum fragmentation component, or
from “fast-slow” recombination?
 Are thermal recomb., “fast-slow” recomb. and vacuum fragmentation treatments compatible? Double-counting, mixing d.o.f., etc.?
 Do coalescence models have predictive power? E.g., can they
predict centrality-dependences?
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T. Hallman SC MTG Jan 2005
Gluon Saturation: a QCD Scale for Initial Gluon Density +
Early Thermaliz’n Mechanism?
Saturation model
curves use optical
Glauber
sNN = 130 GeV
Au+Au
 Does the high initial gluon density inferred from parton E loss fits demand a
deconfined initial state? Can QCD illuminate the initial conditions?
 Assuming initial state dominated by g+g below the saturation scale (constrained by HERA e-p), Color Glass Condensate approaches ~account for
RHIC bulk rapidity densities  dNg/dy ~ consistent with parton E loss.
 How robust is agreement, given optical vs. MC Glauber ambiguity in calcu
-lating Npart , and assumption of ~one charged hadron per gluon?
 CGC applies @ SPS too? If not, why is measured dNch/d(sNN) so smooth?
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T. Hallman SC MTG Jan 2005
Lattice QCD Predicts Some Sort of RAPID Transition!
in entropy density,
hence pressure
The most realistic calcs.  no
discontinuities in
thermodynamic proper-ties @
RHIC conditions (i.e., no 1stor 2nd-order phase transition),
but still crossover transition
with rapid evolution vs.
temperature near Tc 160 –
170 MeV.
in chiral
condensate
in heavy-quark
screening mass
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T. Hallman SC MTG Jan 2005
But What We Observe (at least in the soft sector)
Appears Smooth :
HBT parameters
Charged particle pseudorapidity density
pT-integrated
elliptic flow
pT-integrated elliptic
flow, scaled by initial
spatial eccentricity
No exp’tal smoking gun!  Rely on theory-exp’t comparison
 Need critical evaluation of both! Theory must eventually
explain the smooth energy- and centrality-dependences.
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The Five Pillars of RHIC Wisdom
Ideal hydro
T. Hallman SC MTG Jan 2005
Early thermalization
+ soft EOS
Statistical model
Quark recombination
 constituent q d.o.f.
…suggest appealing QGP-based
picture of RHIC collision evolution, BUT invoke 5 distinct
models, each with own ambiguities, to get there.
pQCD parton E loss
u, d, s equilibration near
Tcrit
CGC
Very high
anticipated
initial gluon
density
Very high
inferred
initial
gluon
density
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T. Hallman SC MTG Jan 2005
Summary on QGP Search
All indications are that a qualitatively new form of matter is being
produced in central AuAu collisions at RHIC
1)
The extended reach in energy density at RHIC appears to reach simplifying
conditions in central collisions -- ~ideal fluid expansion; approx. local thermal
equilibrium.
2)
The Extended reach in pT at RHIC gives probes for behavior inaccessible at lower
energies – jet quenching; ~constituent quark scaling.
But: In the absence of a direct signal of deconfinement revealed by experiment
alone, a QGP discovery claim must rest on the comparison with a theoretical
framework. In this circumstance, further work to establish clear predictive power
and provide quantitative assessments of theoretical uncertainties is necessary for
the present appealing picture to survive as a lasting one.
In order to rely on theory for compelling QGP discovery claim, we
need: greater coherence; fewer adjusted parameters; quantitative
estimates of theoretical uncertainties
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T. Hallman SC MTG Jan 2005
Backup Slides
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T. Hallman SC MTG Jan 2005
Critical Future Exp’t Needs: Short-Term (some
data
already in the bag from run 4)
Establish v2 scaling more definitively: better statistics, more particles
(incl. , , resonances), include  correlations in recomb.-model fits.
Establish that jet quenching is an indicator of parton, not hadron, E
loss: higher pT; better statistics dihadron correlations vs. reaction
plane; away-side punchthrough? charmed meson suppression?
Extend RHIC Au+Au meas’ments down toward SPS energy, search for
possible indicators of a rapid transition in measured properties:
determine turn-on of jet suppression vs. s; pp reference data crucial.
Measure charmonium yields + open
charm yields and flow, to search for
signatures of color screening and
partonic collectivity: charmed hadrons
in chem. equil.? Coalescence vs. fragmentation? D-meson flow; J/ suppression? (eventually  , other “onia”)
Measure hadron correlations with far
forward high-energy hadrons in d+Au:
search for monojet signature of
interaction with classical gluon field.
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T. Hallman SC MTG Jan 2005
Some Critical Future Exp’t Needs: Longer-Term
Develop thermometers for the early stage of the collision, when thermal equilibrium is first
established: direct photons ( HBT for low E), thermal dileptons.
Quantify parton E loss by measurement of mid-rapidity jet fragments tagged by hard direct
photon, a heavy-quark hadron, or a far forward energetic hadron: constrain E loss of light
quarks vs. heavy quarks vs. gluons in bulk matter.
Test quantitative predictions for elliptic flow in U+U collisions:
Considerable extrapolation away from Au+Au  significant
test for hydro predictive power @ RHIC.
Measure hadron multiplicities, yields, correlations and flow at
LHC & GSI, and compare to quantitative predictions based on
models adjusted to work at RHIC: test viability and
falsifiability of QGP-based theoretical framework.
Devise tests for the fate of fundamental QCD symmetries in
RHIC collision matter: chiral & UA(1) restoration? CP
violation? Look especially at the strongly affected particles
opposite a high-pT hadron tag.
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Soft-Hard Correlations: Partial Approach T. Hallman SC MTG Jan 2005
Toward Thermalization?
Leading hadrons
Medium
STAR PRELIMINARY
{
Closed symbols  4 < pTtrig < 6 GeV/c
Open symbols  6 < pTtrig < 10 GeV/c
{
sNN = 200 GeV
Au+Au results:
Assoc. particles:
0.15 < pT < 4 GeV/c
Away side not jet-like! In central Au+Au, the balancing hadrons are
greater in number, softer in pT, and distributed ~statistically [~ cos()]
in angle, relative to pp or peripheral Au+Au.
 away-side products seem to approach equilibration with bulk medium
traversed, making thermalization of the bulk itself quite plausible.
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T. Hallman SC MTG Jan 2005
Five Pieces of Important Evidence
Early thermalization
+ soft EOS
Statistical model
Ideal hydro
u, d, s equilibration
near Tcrit
Quark recombination
 constituent q d.o.f.
pQCD parton E loss
CGC
Very high
anticipated
initial gluon
density
Very high
inferred
initial
gluon
density
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