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

Blyth, D. ; Fry, J. ; Fomin, N. ; Alarcon, R. ; Alonzi, L. ; Askanazi, E. ; Baeßler, S. ; Balascuta, S. ; Barrón-Palos, L. ; Barzilov, A. ; Bowman, J. D. ; Birge, N. ; Calarco, J. R. ; Chupp, T. E. ; Cianciolo, V. ; Coppola, C. E. ; Crawford, C. B. ; Craycraft, K. ; Evans, D. ; Fieseler, C. ; Frle≈æ, E. ; Garishvili, I. ; Gericke, M. T. W. ; Gillis, R. C. ; Grammer, K. B.

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 $\Delta I=1$, $^{3}S_{1}\rightarrow {^{3}P_{1}}$ 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 $C^{^{3}S_{1}\rightarrow ^{3}P_{1}}/C_{0}=[-7.4 \pm 3.5 (stat) \pm 0.5 (sys)] \times 10^{-11}$. We describe the experiment, data analysis, systematic uncertainties, and the implications of the result.

Publication Date:
Dec 14 2018
Date Submitted:
Jun 28 2019
Pagination:
242002
Citation:
Physical Review Letters
121
14
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