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

TL;DR

This paper reports the first measurement of a parity-violating gamma-ray asymmetry in polarized neutron capture on hydrogen, providing insights into weak nucleon-nucleon interactions and pion exchange models.

Contribution

It presents the first experimental observation of a specific P-odd gamma asymmetry in neutron-proton capture, linking it to fundamental weak interaction constants.

Findings

Measured asymmetry $A^{np}_$ = [-3.0  1.4  0.2] imes 10^{-8}

Inferred weak pion-nucleon coupling $h_{}^1$ = [2.6  1.2  0.2] imes 10^{-7}

Determined pionless EFT constant $C^{3S_1 ightarrow 3P_1}/C_0$ = [-7.4  3.5  0.5] imes 10^{-11} MeV^{-1}

Abstract

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.