The sign of the average emission angle of nonequilibrium light particles was determined from the circular polarization of coincident $\gamma$ rays emitted from residual nuclei for ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$-induced reactions on ${}_{}{}^{154}{}_{}{}^{}\mathrm{Sm}$ at $\frac{E}{A}=20\mathrm{} \mathrm{and} 35$ MeV. Light particles ($p,d,t,\alpha$) are preferentially emitted to negative angles. This effect is consistent with deflection of the particles by the attractive nuclear mean field. Numerical solutions of the Boltzmann-Uehling-Uhlenbeck equation demonstrate that the experimental result is sensitive to the interplay between the nuclear mean field and nucleon-nucleon collisions.

• Received 21 March 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.559