We report the first observation of the parity-violating gamma-ray asymmetry $A_{\gamma }^{np}$ in neutron-proton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. $A_{\gamma }^{np}$ isolates the $\mathrm{\Delta }I=1$, ${{}^{3}S}_1\to {{}^{3}P}_1$ 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 effective field theory. We measured $A_{\gamma }^{np}=[-3.0\pm 1.4(\mathrm{stat})\pm 0.2(\mathrm{syst})]\times {10}^{-8}$, which implies a DDH weak $\pi NN$ coupling of $h_{\pi }^1=[2.6\pm 1.2(\mathrm{stat})\pm 0.2(\mathrm{syst})]\times {10}^{-7}$ and a pionless EFT constant of $C^{{{}^{3}S}_1\to {{}^{3}P}_1}/C_0=[-7.4\pm 3.5(\mathrm{stat})\pm 0.5(\mathrm{syst})]\times {10}^{-11}{\mathrm{MeV}}^{-1}$. We describe the experiment, data analysis, systematic uncertainties, and implications of the result.

• Received 11 March 2018
• Revised 22 October 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.242002