Chemical potentials ofπ−andπ+in heavy-ion collisions

We consider a chemical nonequilibrium model to describe the pion production in Ar+KCl and La+La collisions at initial energies ${\mathit{E}}_{\mathrm{lab}}$=0.5–1.8 GeV/nucleon. The excess of low-energy ${\mathrm{\pi }}^{\mathrm{-}}$ is interpreted as the manifestation of positive chemical potential of ${\mathrm{\pi }}^{\mathrm{-}}$ at the thermal freeze-out. We find that in collisions between nuclei with large atomic numbers the chemical potential of ${\mathrm{\pi }}^{+}$ is smaller than that of ${\mathrm{\pi }}^{\mathrm{-}}$. This leads to the prediction of much less excess of low-energy ${\mathrm{\pi }}^{+}$, than as measured in the ${\mathrm{\pi }}^{\mathrm{-}}$ case, in heavy-ion collisions at bombarding energies in the region of 1 GeV/nucleon.