Probing nuclear expansion dynamics with $π^-/π^+$-spectra

Abstract

We study the dynamics of charged pions in the nuclear medium via the ratio of differential $π^-$- and $π^+$-spectra in a coupled-channel BUU (CBUU) approach. The relative energy shift of the charged pions is found to correlate with the pion freeze-out time in nucleus-nucleus collisions as well as with the impact parameter of the heavy-ion reaction. Furthermore, the long-range Coulomb force provides a 'clock' for the expansion of the hot nuclear system. Detailed comparisons with experimental data for $Au + Au$ at 1 GeV/A and $Ni + Ni$ at 2.0 GeV/A are presented.

21 pages, latex, figures included