Universal hydrodynamics and charged hadron multiplicity at the LHC

Abstract

Time evolution of a "little bang" created in heavy ion collisions can be divided into two phases, the pre-equilibrium and hydrodynamic. At what moment the evolution becomes hydrodynamic and is there any universality in the hydrodynamic flow? To answer these questions we briefly discuss various versions of hydrodynamics and their applicability conditions. In particular, we elaborate on the idea of "universal" (all-order resumed) hydrodynamics and propose a simple new model for it. The model is motivated by results obtained recently via the AdS/CFT correspondence. Finally, charged hadron multiplicities in heavy ion collisions at the RHIC and LHC are discussed. At the freezout, the multiplicities can be related to total entropy produced in the collision. Assuming the universal hydrodynamics to hold, we calculate the entropy production in the hydro stage of the collision. We end up speculating about a connection between the multiplicity growth and the temperature dependence of the QGP viscosity.