extreme states of matter and ultra-relativistic heavy ion
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Extreme states of matterand
ultra-relativistic heavy ion collisions
Jean-Paul Blaizot, IPhT- Saclay CEA & CNRS
Rencontres du VietnamQui NhonVietnam
20/12/2011
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The QCD phase diagram
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Colliding heavy nuclei ! Initial conditions. Fluctuations (geometry,
nucleus wave function and its parton content)
Particle (entropy) production. Involves mostly small x partons
Thermalization. Quark-gluon plasma. Hydrodynamical expansion
Hadronization. Hydrodynamic expansion continues till freeze-out. Apparent chemical equilibrium at freeze-out
(x = p⊥/√s ∼ 10−2 − 10−4 for p⊥ � 2GeV)
One characteristic scale: saturation momentum Qs
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Main surprises from RHIC
(confirmed by LHC)
concern matter before freeze-out
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(STAR: Phys.Rev.Lett.91:072304,2003)
Matter is opaque to the propagation of jets
The jet produced near the surface ‘escapes’ normally. Its partner is absorbed in the produced plasma.
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Produced matter flows like a fluid
(Quark-Matter 02)
(ALICE QM‘2011)
The flow takes place preferentially in the reaction plane rather than away from it.
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The flow is sensitive to initial nuclear density fluctuations
peripheral central
J. Velkowska,QM2011and Luzum, arXiv:1011:5173
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(Luzum, Romatschke, 2007)
The low viscosity of the quark-gluon plasma
peripheral central
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Viscosity at weak and strong coupling
(Policastro, Son, Starinet, 2001)
η
s=14π�
kB
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Small and short equilibration time seem incompatible with weak coupling
Theoretical puzzle
αs ∼ 0.3 ÷ 0.4
η/s
However
- the coupling is not huge ( )
- our present understanding of initial stages of heavy ion collisions is based on weak coupling (for asymptotically large nuclei and large energies)
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High density partonic systems
At saturation, occupationnumbers are large
xG(x,Q2)πR2Q2s
∼ 1αs
Description in terms ofclassical fields possible(CGC, etc)
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The overpopulated quark-gluon plasma
Initial conditions
�0 = �(τ = Q−1s ) ∼Q4sαs
n0 = n(τ = Q−1s ) ∼Q3sαs
�0/n0 ∼ Qs
n0 �−3/40 ∼ 1/α1/4s
�eq ∼ T 4 neq ∼ T 3
Mismatch by a large factor (at weak coupling)
Overpopulation parameter
In equilibrated quark-gluon plasma
α−1/4s
(t0 ∼ 1/Qs)
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Formation of a Bose-Einstein condensate?
As a result of their interactions, gluons acquire a ‘mass’, and can condense.
Note: when all dependence on the coupling constant disappears from kinetic equations
f ∼ 1/αs
(J.-P. B, F. Gelis, J. Liao, L. McLerran and R. Venugopalan, arXiv:1107.5296)
∂t f = C[ f ] ∼ α2 f 3 ∼1α
�f ∼ 1α
�
Evidences for this phenomenon in classical scalar field theories (Epelbaum, Gelis, NPA 872 (2011) 210 ). Non abelian gauge theories ?
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Conclusions
- the field of ultra-relativistic heavy ion collisions is a very rich one, it addresses fundamental issues (hot and dense QCD matter, high density partonic systems, etc)
- exciting developments in recent years, both experimentally and theoretically (RHIC, LHC; CGC, AdS/CFT, etc), many open questions/puzzles (weak vs strong coupling, thermalization, etc)
- future of the field looks bright, with many facilities allowing for such studies: LHC, RHIC2, FAIR, NICA