Measurement of the parity violating asymmetry Aγ in n⃗+p→d+γ

The weak pion-nucleon coupling constant Hπ1 remains poorly determined, despite many years of effort. The recent measurement of the 133Cs anapole moment has been interpreted to give a value of Hπ1 almost an order of magnitude larger than the limit established in the 18F parity doublet experiments. A measurement of the gamma ray directional asymmetry Aγ for the capture of polarized neutrons by hydrogen has been proposed at Los Alamos National Laboratory. This experiment will determine Hπ1 independent of nuclear structure effects. However, since the predicted asymmetry is small, Aγ≈5×10−8, systematic effects must be reduced to <5×10−9. The design of the experiment will is presented, with an emphasis on the techniques used for controlling systematic errors.