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The study of hot and dense matter at RHIC
RIKEN, TokyoDecember 6, 2004
Jean-Paul Blaizot, CNRS and ECT*
QCD and asymptotic freedom
Matter is simple at high density/high temperature- early universe- deconfined quark-gluon plasma
Interactions beween « strongly » interacting particles arecalculable
Weak coupling, few particles
Factorisation theorems
Accurate calculations can be done
Perturbative QCD
What can we predict from pQCD?
Initial production of hard probes (jets, heavy quarks, etc.)
Deviations from predictions signal new physics (e.g., J/Psisuppression (?), jet quenching, heavy quark propagation,etc.)
In A-A collisions, binary scaling
Initial wave function
Elementary cross-section
Fragmentation function
Factorization theorems of pQCD
At high temperature and/or high density matter is « simple »
But transition from hadronic phase to the asymptotic quark-gluon plasma is not
Matter at high temperature is like radiation
(from F. Karsch, hep-lat/0106019)
Weakly interacting quasiparticles
Kazmarek et al hep-lat/0312015
Heavy quark free energy
Laermann et al, NPB42(1995)
Color averaged potential
Expected « signatures » of the QGP
- Stangeness enhancement- J/Psi suppression- Direct signals of phase transiton (thresholdphenomena, characteristic collective behaviours)- Jet energy loss- Thermal radiation- Etc.
Phenomena related to all of those have been observed
Experimental discoveriesMany evidences for non trivial phenomena
Large energy density
Collective behaviour
Suppression of charmonium bound states
Jet quenching and strong « final state » interactions
Hints of gluon saturation
And much more…
Statistical/chemical equilibrium at freeze-out
Collective flow of produced matterProduced particles flow preferentiallyin the reaction plane
(J.-Y. Ollitrault, 1992)
€
εx =y 2 − x 2
y 2 + x 2
€
εp =Txx −Tyy
T xx + Tyy(P.F.Kolb, J. Sollfrank and U. Heinz, PRC 62 (2000) 054909)
J/ψ SUPPRESSION IN NUCLEAR COLLISIONS
(from N. Willis)
Expected
Measured
Evidence for the formation of the quark-gluon plasma (?)
« …a strongly interacting medium is created with extremely high energy density, whose description in terms of hadronic degrees of freedom is inappropriate… » (PHOBOS white paper)
RHIC 2004
« Are we there yet? » (STAR white paper)
« …a new state of matter has beencreated…at energy densities whichexceed by more than a factor 20 that ofnormal nuclear matter….featuresmany of the characteristics of thetheoretically predicted quark-gluonplasma. » (Heinz & Jacob)
SPS 2000
(Indirect) evidences for collective behaviour at thepartonic level
Evidence for short thermalization time, low viscosity, strong interaction at early times produced matter is not an ideal quark-gluon plasma
No sharp transition in any observable as a function of energy
At the SPS, many phenomena obscured by strong final state interactions, particle production dominated by soft physics
Initial state plays a more important role at RHIC
Phenomena more spectacular at RHIC, interpretation reliesmore on QCD
Gluon saturation a regime with weak coupling, many cooperating degrees of freedom, strong fields.
Saturation scale
Non linear effects important when
Gribov, Levin, Ryskin 83Mueller, Qiu 86
The saturation scale
In a nucleus
The conditions in the central rapidity of a nucleus-nucleus collision at RHIC are similar to those at HERA
From fit to DIS (HERA)
Early stages of a nucleus-nucleus collision
Partons set free have typical tranverse momenta
They are set free at (proper) time
At that time
(J.-P. B., A. Mueller, 87)Momentum distributions
COLOR GLASS CONDENSATE AND JIMWLK(*) EQUATION
(*) Jalilian-Marian, Iancu, McLerran, Weigert, Leonidov, Kovner
NON LINEAR EVOLUTION EQUATIONS
BALITSKY-KOVCHEGOV EQUATION
(Gotsman, Levin, Lublinsky, Maor hep-ph/0209074)
Numerical solution of the BK equation + DGLAP corrections
« Limiting fragmentation »
PHOBOS, PRL 91
Interpretation in the color glass picture: J. Jalilian-Marian, nucl-th/0212018
Albacete et al, hep-ph/0307179
y0
0.05
0.1
0.2
0.4
0.6
1
1.4
2
(Related analytical work by Iancu, Itakura, Triantafyllopoulos hep-ph/0403103)
SUMMARY- Vast body of precise data, provide detailed information oncollision dynamics, suggesting a « standard scenario » ofnucleus-nucleus collisions. However, no unique frameworkaccounts for all the data. Puzzles remain- Strongly interacting matter is produced in high energynucleus-nucleus collisions- Many evidences that partonic degrees of freedom play animportant role in the collision dynamics at RHIC, while SPSpysics is largely dominated by « soft » physics- Early stages of the collisions, and hence « intial state effects »play an important role at RHIC (and will be more so at LHC).Hints of saturation (color glass condensate) may be alreadypresent at RHIC- QCD has become a central reference in the analysis of the data