Mass dependence of the disappearance of flow in nuclear collisions

The disappearance of collective flow in nucleus-nucleus collisions occurs at an incident energy (${\mathit{E}}_{\mathrm{bal}}$) where the attractive scattering dominant at low energies balances the repulsive scattering dominant at high energies. We have performed the first systematic study of the entrance-channel mass dependence of the disappearance of flow and hence ${\mathit{E}}_{\mathrm{bal}}$. The new data presented for the C+C, Ne+Al, Ar+Sc, and Kr+Nb systems show that ${\mathit{E}}_{\mathrm{bal}}$ scales as ${\mathit{A}}^{\mathrm{\ensuremath{-}}1/3}$ where A is the mass of the combined system. Boltzmann-Uehling-Uehlenbeck model calculations show trends which are in qualitative agreement with these new results.