Precise determination of the spin-transfer coefficient KNN′ for n⃗p elastic scattering at 187 MeV

We have measured the normal-component spin-transfer observable KNN′ for np elastic scattering to a precision of ±0.02 at 15 angles in the far-backward region (105 °⩽θcm⩽172 °) at a neutron bombarding energy of 187 MeV. This kinematic regime is of particular interest in that the scattering is mediated predominantly through exchange of a single charged pion, and hence is expected to depend sensitively on the strength of the π±NN coupling constant gc2, a quantity whose precise value remains highly controversial. This controversy has been heightened by new np absolute cross section data, also taken at back angles, which appear to favor the higher values of gc2 obtained in earlier partial-wave analyses, but disagree with most recent predictions. After reviewing the current situation, I will describe the experimental apparatus used in our measurement and our data acquisition and analysis procedures, paying particular attention to potential sources of systematic error and their effect on the data. Our primary r...