We report a measurement of two energy-weighted gamma cascade angular distributions from polarized slow neutron capture on the Cl-35 nucleus, one parity-odd correlation proportional to (s) over right arrown.(k) over right arrow (gamma) and one parity-even correlation proportional to (s) over right arrown.(k) over right arrown x (k) over right arrow (gamma). A parity-violating asymmetry can appear in this reaction due to the weak nucleon-nucleon interaction, which mixes opposite parity S-and P-wave levels in the excited compound Cl-36 nucleus formed upon slow neutron capture. If parity-violating (PV) and parity-conserving (PC) terms both exist, the measured differential cross section can be related to them via d sigma sigma/d Omega proportional to 1 +A(gamma,PV) cos theta +A(gamma,PC) sin theta. The PV and PC asymmetries for energy-weighted gamma cascade angular distributions for polarized slow neutron capture on Cl-35 averaged over the neutron energies from 2.27-9.53 meV were measured to be A(gamma,PV) = (-23.9 +/- 0.7) x 10(-6) and A(gamma,PC) = (0.1 +/- 0.7) x 10(-6). These results are consistent with previous experimental results. Systematic errors were quantified and shown to be small compared to the statistical error. These asymmetries in the angular distributions of the gamma rays emitted from the capture of polarized neutrons in Cl-35 were used to verify the operation and data analysis procedures for the NPDGamma experiment, which measured the parity-odd asymmetry in the angular distribution of gamma s from polarized slow neutron capture on protons.

Neutron beta decay is one of the most fundamental processes in nuclear physics and provides sensitive means to uncover the details of the weak interaction. Neutron beta decay can evaluate the ratio of axial-vector to vector coupling constants in the standard model, λ = gA/gV, through multiple decay correlations. The Nab experiment will carry out measurements of the electron-neutrino correlation parameter a with a precision of δa/a = 10−3 and the Fierz interference term b to δb = 3 × 10−3 in unpolarized free neutron beta decay. These results, along with a more precise measurement of the neutron lifetime, aim to deliver an independent determination of the ratio λ with a precision of δλ/λ = 0.03% that will allow an evaluation of Vud and sensitively test CKM unitarity, independent of nuclear models. Nab utilizes a novel, long asymmetric spectrometer that guides the decay electron and proton to two large area silicon detectors in order to precisely determine the electron energy and an estimation of the proton momentum from the proton time of flight. The Nab spectrometer is being commissioned at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source at Oak Ridge National Lab. We present an overview of the Nab experiment and recent updates on the spectrometer, analysis, and systematic effects.

We report the first observation of the parity-violating gamma-ray asymmetry A_{γ}^{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_{γ}^{np} isolates the ΔI=1, ^{3}S_{1}→^{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 effective field theory. We measured A_{γ}^{np}=[-3.0±1.4(stat)±0.2(syst)]×10^{-8}, which implies a DDH weak πNN coupling of h_{π}^{1}=[2.6±1.2(stat)±0.2(syst)]×10^{-7} and a pionless EFT constant of C^{^{3}S_{1}→^{3}P_{1}}/C_{0}=[-7.4±3.5(stat)±0.5(syst)]×10^{-11}  MeV^{-1}. We describe the experiment, data analysis, systematic uncertainties, and implications of the result.

The Nab experiment will measure the correlation a between the momenta of the beta particle and antineutrino in neutron decay as well as the Fierz term b which distorts the beta spectrum. The a correlation can be used to extract the quark mixing matrix element V-ud, which is used to precisely test our understanding of the electroweak interaction. A non-zero Fierz term would signal the presence of scalar and tensor currents which must come from new physics beyond the Standard Model of Particle Physics. Nab uses an asymmetric spectrometer to determine the proton time-of-light and the beta energy in order to reconstruct the angular correlation between the electron and antineutrino. The charged particles are detected using thick, large-area, segmented silicon detectors, which must be held at a potential of -30 kV to detect the very low energy protons. A 24 channel prototype system has been tested and found to meet specifications of 3 keV full-width half maximum (FWHM) energy resolution at 30 keV, 50 ns signal rise times, and thresholds < 10 keV. The 128 ch fully instrumented detection system is currently in development. Silicon drift detectors in development by the TRISTAN collaboration can reach an energy resolution as low as 135 eV FWHM at 6 keV, and could enable the next generation of neutron beta decay experiments.