Charge-Symmetry Breaking in Neutron-Proton Elastic Scattering

The effects of spin-dependent charge-symmetry-breaking forces can be measured in elastic $n-p$ scattering. We find that a term in the one-pion exchange potential arising from the neutron-proton mass difference is dominant. This, together with single-photon exchange, gives a result in agreement with a recent precise measurement made at TRIUMF. Other shorter-ranged effects associated with rho exchange, meson mixing, two-pion exchanges, and quark interactions give smaller contributions.