| Summary: | We report the first observation of the parity-violating gamma-ray asymmetry A[subscript γ under superscript 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[subscript γ under superscript np] isolates the ΔI=1, ³S[subscript 1]→³P[subscript 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[subscript γ under superscript np]=[-3.0±1.4(stat)±0.2(syst)]×10-⁸, which implies a DDH weak πNN coupling of h[subscript π under superscript 1]=[2.6±1.2(stat)±0.2(syst)]×10-⁷ and a pionless EFT constant of [physics equation; see resource]. We describe the experiment, data analysis, systematic uncertainties, and implications of the result. ©2018
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