Speaker
Description
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. The asymmetry isolates the long-range component of the hadronic weak interaction, corresponding to the $ \Delta I = 1$, $^3S_1 \rightarrow ~^{3}P_1$ component of the weak nucleon-nucleon interaction. Weak $NN$ interaction observables in few nucleon systems are currently calculated, using modern effective field theory, the $1/N_c$ expansion formalism of QCD, and lattice gauge theory. We measured $A^{np}_{\gamma} = \left(-3.0 \pm 1.4~\rm{(stat.)} \pm 0.2~\rm{(sys.)}\right) \times 10^{-8}$, which implies a DDH weak $\pi NN$ coupling of $h^1_\pi = \left(3.1 \pm 1.5\right)\times 10^{-7}$ and a pion-less EFT constant of $C^{^3S_1 \rightarrow ^3P_1}/C_0 = \left(-5.2 \pm 2.4\right)\times 10^{-7} ~\rm{MeV^{-1}}$. We describe the experiment, data analysis, systematic uncertainties, and the implications of the result.
