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

We report the first observation of the parity-violating 2.2 MeV gamma-ray asymmetry $A^{np}_gamma$ in neutron-proton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. $A^{np}_gamma$ isolates the $Delta I=1$, mbox{$^{3}S_{1}rightarrow {^{3}P_{1}}$} 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 $A^{np}_gamma = [-3.0 pm 1.4 (stat) pm 0.2 (sys)]times 10^{-8}$, which implies a DDH weak $pi NN$ coupling of $h_{pi}^{1} = [2.6 pm 1.2(stat) pm 0.2(sys)] times 10^{-7}$ and a pionless EFT constant of $C^{^{3}S_{1}rightarrow ^{3}P_{1}}/C_{0}=[-7.4 pm 3.5 (stat) pm 0.5 (sys)] times 10^{-11}$ MeV$^{-1}$. We describe the experiment, data analysis, systematic uncertainties, and the implications of the result.