First Observation of P-odd γ Asymmetry in Polarized Neutron Capture on Hydrogen.

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 Delta I = 1, S-3(1)-> P-3(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 +/- 1.4(stat) +/- 0.2(syst)] x 10(-8), which implies a DDH weak pi NN coupling of h(pi)(1) = [2.6 +/- 1.2(stat) +/- 0.2(syst)] x 10(-7) and a pionless EFT constant of C-3S1 -> 3P1/C-0 = [-7.4 +/- 3.5(stat) +/- 0.5(syst)] x 10(-11) MeV-1. We describe the experiment, data analysis, systematic uncertainties, and implications of the result.