First Observation of $P$-odd ${\hat{I}}^3$ Asymmetry in Polarized Neutron Capture on Hydrogen

Abstract

We report the first observation of the parity-violating 2.2 MeV gamma-ray asymmetry $A_{\hat{I}}^3$ in neutron-proton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. $A_{\hat{I}}^3$ isolates the $\hat{I}”I=1$, \mbox{${}^3{S}_1\to {}^3{P}_1$} 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 EFT. We measured $A_{\hat{I}}^3=[-3.0\pm 1.4(stat)\pm 0.2(sys)]\times 10^{-8}$, which implies a DDH weak $\ddot{I}€NN$ coupling of $h_{\ddot{I}}{€}^1=[2.6\pm 1.2(stat)\pm 0.2(sys)]\times 10^{-7}$ and a pionless EFT constant of $C^{{}^3{S}_1{\to }^3{P}_1}/C_0=[-7.4\pm 3.5(stat)\pm 0.5(sys)]\times 10^{-11}$ MeV${}^{-1}$. We describe the experiment, data analysis, systematic uncertainties, and the implications of the result.

6 pages, 5 figures