Parity violation in threshold neutron-proton scattering

Parity violation in weak neutron-proton scattering is studied at threshold where neutron spin rotation might occur. The authors use the weak nucleon-nucleon interaction suggested by Desplanques, Donoghue and Holstein (1980) together with the Paris or Reid-soft-core forms of the strong interaction. Attention is focused on the contribution of various spin-isospin operators in the weak interaction as well as on the effect of distortion by S- and P-wave strong amplitudes. The spin rotation angle is found to be -8.84*10^-9 rad cm^-1 (using the Paris potential). In addition to the neutron spin rotation, there should also be an asymmetry in the total cross section (between positive- and negative-helicity states) due to the occurrence of the inelastic capture channel n+p to d+ gamma . Rough estimates of this quantity indicate that it is negligibly small.