Bjorken Expansion with Gradual Freeze Out

Received (received date) Revised (revised date) The freeze out of the expanding systems, created in relativistic heavy ion collisions, will be discussed. We combine kinetic freeze out equations with Bjorken type system expansion into a unified model. Such a model is a more physical generalization of the earlier simplified non-expanding freeze out models. We shall see that the basic freeze out features, pointed out in the earlier works, are not smeared out by the expansion. 1. Model for the simultaneously expanding and freezing out fireball In this paper we present a simple freeze out (FO) model, which describes the FO of particles from a Bjorken expanding fireball 1. The important feature of the proposed scenario is that physical freeze out is completely finished in a finite time, which can be varied from 0 (FO hypersurface) to ∞. In the other words our FO happens in a layer, i.e. in a domain restricted by two parallel hypersurfaces τ = τ 1 and τ = τ 1 + L, where τ is the proper time variable and L is the maximal duration of the FO process. The present model simultaneously describes the gradual FO and the expansion of the system, and thus it is a more realistic extension of the oversimplified FO models, which did not include system expansion 2,3,4,5. In Ref. 3 authors have also adopted kinetic gradual FO model to Bjorken geometry, but combined it with