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