Evolution of Anisotropy of Parton System from Relativistic Heavy-Ion Collisions

Evolution of anisotropy in momentum and coordinate space of the parton system produced in relativistic heavy-ion collisions is discussed within the free-streaming approximation. The momentum distribution evolves from the prolate shape - elongated along the beam - to the oblate one - squeezed along the beam. At the same time the eccentricity in the coordinate space, which occurs at finite values of impact parameter, decreases. It is argued that the parton system reaches local thermodynamic equilibrium before the momentum distribution becomes oblate.