Mass Dependence of Directed Collective Flow

Sidewards directed fragment flow has been extracted for ${}^{84}\mathrm{Kr}{+}^{197}\mathrm{Au}$ collisions at $E/A=200\mathrm{}\mathrm{MeV}$, using techniques that are free of reaction plane dispersion. The fragment flow per nucleon increases with mass, following a thermal or coalescencelike behavior, and attains roughly constant limiting values at $4\le A\le 12$. Comparisons of the impact parameter dependences of the measured coalescence-invariant proton flow to Boltzmann-Uehling-Uhlenbeck calculations clearly favor a momentum dependent nuclear mean field.