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Register a new userFirst Observation of $P$-odd $gamma$ Asymmetry in Polarized Neutron Capture on Hydrogen
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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.
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We report the first precision measurement of the parity-violating asymmetry in the direction of proton emission with respect to the neutron spin, in the reaction $^{3}mathrm{He}(mathrm{n},mathrm{p})^{3}mathrm{H}$, using the capture of polarized cold neutrons in an unpolarized active $^3rm{He}$ target. The asymmetry is a result of the weak interaction between nucleons, which remains one of the most poorly understood aspects of electro-weak theory. The measurement provides an important benchmark for modern effective field theory (EFT) calculations. Measurements like this are necessary to determine the spin-isospin structure of the hadronic weak interaction. Our asymmetry result is $A_{PV} = left( 1.58 pm 0.97 ~mathrm{(stat)} pm 0.24~mathrm{(sys)}right)times10^{-8}$, which has the smallest uncertainty of any parity-violating asymmetry measurement so far.
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