First Observation of $P$-odd $\gamma$ Asymmetry in Polarized Neutron Capture on Hydrogen

Abstract

We report the first observation of the parity-violating 2.2 MeV 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, 3S1→3P1 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 EFT. We measured Aγnp=[−3.0±1.4(stat)±0.2(sys)]×10−8, which implies a DDH weak πNN coupling of hπ1=[2.6±1.2(stat)±0.2(sys)]×10−7 and a pionless EFT constant of C3S1→3P1/C0=[−7.4±3.5(stat)±0.5(sys)]×10−11. We describe the experiment, data analysis, systematic uncertainties, and the implications of the result.