First Observation of P -odd γ Asymmetry in Polarized Neutron Capture on Hydrogen

We report the first observation of the parity-violating gamma-ray asymmetry Aγnp in neutron-proton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. Aγnp isolates the ΔI=1, S31→P31 component of the weak nucleon-nucleon interaction, which is dominated by pion exchange and can be directly related to a single coupling constant in either the DDH meson exchange model or pionless effective field theory. We measured Aγnp=[-3.0±1.4(stat)±0.2(syst)]×10-8, which implies a DDH weak πNN coupling of hπ1=[2.6±1.2(stat)±0.2(syst)]×10-7 and a pionless EFT constant of CS31→P31/C0=[-7.4±3.5(stat)±0.5(syst)]×10-11 MeV-1. We describe the experiment, data analysis, systematic uncertainties, and implications of the result.