Collective phenomena in the early stages of relativistic heavy-ion   collisions
  A recently developed framework of highly-anisotropic and strongly-dissipative hydrodynamics -- ADHYDRO -- has been introduced and used to analyze the space-time evolution of matter produced in ultra-relativistic heavy-ion collisions. The main goal of this analysis was to study the effect of initial highly-anisotropic stages on the final soft hadronic observables typically measured in the experiment. The study was done in the context of the heavy-ion measurements performed at RHIC (Relativistic Heavy Ion Collider) in Brookhaven National Laboratory. Starting from the general assumption about the form of the phase-space distribution function, thermodynamic properties of locally anisotropic systems of particles have been studied and the form of the generalized equation of state has been formulated. The dynamic equations determining the evolution of a highly-anisotropic fluid have been introduced. The form of the entropy source related to the mechanisms leading to thermalization of the system has been defined. In the simplest case of purely-longitudinal and boost-invariant expansion, different features of the model have been analyzed. Using the ADHYDRO model in the general (3+1)D and boost-invariant (2+1)D versions, different possible scenarios of early stages of heavy-ion collisions have been analyzed. The results of the ADHYDRO model have been compared to the results obtained from the reference, perfect-fluid hydrodynamic model -- LHYQUID. The results of the hydrodynamic models were coupled to the statistical Monte-Carlo model THERMINATOR. The following conclusions have been drawn: a) All studied observables are almost insensitive to the initial anisotropic stage provided the initial conditions of the evolution are properly readjusted, b) Complete thermalization of matter may take place only at the times of about 1 fm/c. In this way the early thermalization puzzle may be circumvented. 
Radoslaw Ryblewski
nucl-th hep-ph