Strangeness evolution in the central region of a heavy-ion collision with transverse flow effects

Abstract

Strangeness evolution in the central region of an ultra-relativistic nucleus-nucleus collision is studied by using a numerically computed boost invariant cylindrically symmetric hydrodynamic flow. Kinetic equations describing the evolution through quark-gluon plasma, mixed and hadron phases are derived. With reasonable collision terms the conclusion is that the strangeness density (=weighted sum of the strange quark density and the sum of K − and K̄ 0 densities) evolves close to or up to a factor 2 below the equilibrium density until the end of the mixed phase. With transverse expansion the dilution of matter in hadron phase is so fast that strangeness creation and annihilation process effectively decouple. This implies that the observed N K / N π is rather independent of the assumed decoupling temperature and lies in the range 0.15-0.3.