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, ^3S_1-> ^3P_1 component of the weak nucleon-nucleon interaction, which is dominated by pion exchange and can be directly related to the weak coupling constant h^1_pi in the DDH meson exchange model and to a low energy constant C^[^3S_1 ->^3P_1]/C_0 in pionless effective fi eld theory. We measured A^(np)_gamma = (-3.0 +/-1.4(stat.)+/-0.2(sys.))x10e-8, which implies a DDH weak NN coupling of h^1_pi = (3.1+/-1.5)x10e-7 and a pionless EFT constant of C^[^3S_1->^3P_1]/C_0 = (-5.2+/-2.4)x10e-10 MeV^(-1). We describe the experiment, data analysis, systematic uncertainties, and the implications of the result.
