Abstract
The freeze out of the expanding systems, created in relativistic heavy ion collisions, is discussed. We combine kinetic freeze out equations with Bjorken type system expansion into a unified model. 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 (freeze out hypersurface) to ∞. The dependence of the post freeze out distribution function on the freeze out time will be studied. As an example, model is completely solved and analyzed for the gas of pions. We shall see that the basic freeze out features, pointed out in the earlier works, are not smeared out by the expansion of the system. The entropy evolution in such a scenario is also studied.
Highlights
In the ultrarelativistic heavy ion collisions at RHIC the total number of the produced particles exceeds several thousands, one can expect that the produced system behaves as a “matter” and generates collective effects
Strong collective flow patterns have been measured at RHIC, which suggests that the hydrodynamical models are well justified during the intermediate stages of the reaction: from the time when local equilibrium is reached until the freeze out (FO), when the hydrodynamical description breaks down
In simulations FO is usually described in two extreme ways: either FO on a hypersurface with zero thickness, or FO described by volume emission model or hadron cascade, which require an infinite time and space for a complete FO
Summary
In the ultrarelativistic heavy ion collisions at RHIC the total number of the produced particles exceeds several thousands, one can expect that the produced system behaves as a “matter” and generates collective effects. In simulations FO is usually described in two extreme ways: either FO on a hypersurface with zero thickness, or FO described by volume emission model or hadron cascade, which require an infinite time and space for a complete FO At first glance it seems that one can avoid troubles with FO modeling using hydro + cascade two module model [1], since in hadron cascades gradual FO is realized automatically. [8] authors have adopted kinetic gradual FO model to Bjorken geometry, but combined it with Bjorken expansion on the consequent, not on the parallel basis: system expands according to Bjorken hydro scenario, but when it reaches beginning of the FO process system stops expansion and gradually freezes out in a fixed volume It was shown in [8] that such a scenario is not physical: the simultaneous modeling of expansion and freeze out is required in order to avoid decreasing total entropy.
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