PARTIAL THERMALIZATION IN ULTRARELATIVISTIC HEAVY-ION COLLISIONS

The approach to local thermal equilibrium of pion-like excitations in the central region of a heavy-ion collision is explored using kinetic theory. We show that rarefaction in the longitudinally expanding system reduces the scattering frequency with time, preventing the fluid from reaching local equilibrium. Instead, a novel partially thermalized flow arises in which the degree of thermalization is determined by the rapidity density dN/dy. We estimate the degree of thermalization of SPS energies.