Measurement of the parity violating asymmetry A_γ in n+p→d+γ

The weak interaction between neutrons and protons has never been resolved experimentally. In analogy with the strong NN interaction, the weak NN interaction at low energy can be parametrized in terms of a meson exchange model with parity violating meson-nucleon couplings. Unlike the measured proton-proton weak interaction, the neutron-proton weak interaction is sensitive to the weak pion-nucleon coupling constant H_π¹. This coupling, which is responsible for the longest-ranged part of the weak NN interaction and is therefore an essential part of any description of weak interactions in nuclei, remains undetermined despite many years of effort. A measurement of the gamma ray directional asymmetry A_γ in the capture of polarized neutrons by parahydrogen has been proposed at Los Alamos National Laboratory. The goal of this experiment is to determine A_γ with a relative standard uncertainty of <5×10^-9, which is smaller than all modern predictions for the size of the asymmetry. The design of the experiment is presented, with an emphasis on the techniques used for controlling systematic errors.