Systematic Investigation of Negative Cooper-Frye Contributions in Heavy Ion Collisions Using Coarse-grained Molecular Dynamics

摘要

In most heavy ion collision simulations involving relativistic hydrodynamics,the Cooper-Frye formula is applied to transform the hydrodynamical fields toparticles. In this article the so-called negative contributions in theCooper-Frye formula are studied using a coarse-grained transport approach. Themagnitude of negative contributions is investigated as a function of hadronmass, collision energy in the range of $E_{\rm lab} = 5$--$160A$ GeV, collisioncentrality and the energy density transition criterion defining thehypersurface. The microscopic results are compared to negative contributionsexpected from hydrodynamical treatment assuming local thermal equilibrium. The main conclusion is that the number of actual microscopic particles flyinginward is smaller than the negative contribution one would expect in anequilibrated scenario. The largest impact of negative contributions is found tobe on the pion rapidity distribution at midrapidity in central collisions. Forthis case negative contributions in equilibrium constitute 8--13\% of positivecontributions depending on collision energy, but only 0.5--4\% in cascadecalculation. The dependence on the collision energy itself is found to benon-monotonous with a maximum at 10-20$A$ GeV.