Nuclear effects in leading neutron production

Absorptive corrections, known to suppress proton-neutron transitions with large fractional momentum $z\to1$ in pp collisions, become dramatically strong on a nuclear target, and push the partial cross sections of leading neutron production to the very periphery of the nucleus. The mechanism of $\pi$-$a_1$ interference, which successfully explains the observed single-spin asymmetry in polarized $pp\to nX$, is extended to collisions of polarized protons with nuclei. Corrected for nuclear effects, it explains the observed single-spin azimuthal asymmetry of neutrons, produced in inelastic events, where the nucleus violently breaks up. The single-spin asymmetry is found to be negative and nearly $A$-independent.