Session JB: Invited Session: New Results In Parity Violation

The Parity-Violating Directional Asymmetry in the Capture of Polarized Neutrons on Protons

The NPDGamma experiment completed 2 years of data taking as of December 2014. The experiment was done at the Fundamental Physics Cold Neutron Beam at Spallation Neutron Source. The apparatus consisted of a supermirror polarizer, a resonant spin rotator, a 17-liter liquid para hydrogen target, an array of 48 CsI gamma detectors, and a current-mode data acquisition system. The experiment has measured the parity-violating asymmetry between the polarization direction of neutrons and the direction of the gamma ray in the reaction $\vec{n} + p \to ^2d + \gamma$. This asymmetry determines the part of the hadronic weak interaction carried by the pion. We will present a value for the asymmetry and the weak pion-nucleon coupling. We describe auxiliary experiments to determine the neutron polarization, geometrical factors that describe the sensitivity of each detector to the asymmetry, and other quantities. We describe the analysis procedures and algorithms. We discuss statistical and systematic uncertainties. The statistical uncertainty is consistent with the counting statistics limit. The combined systematic uncertainty from all sources is small compared to the statistical uncertainty.   

aCORN: A Measurement of the Beta-Antineutrino Correlation in Neutron Decay

The aCORN experiment has measured the electron-antineutrino angular correlation coefficient (the ``a'' coefficient) in free neutron decay. aCORN uses the dependence of the recoil proton momentum on the opening angle between the electron and the neutrino to form an asymmetry. The apparatus accepts decays where the antineutrino is restricted to two momentum groups having equal solid angle. In this geometry, proton time of flight distinguishes between decays with a large or small opening angle between the electron and the antineutrino. The correlation coefficient is determined from the asymmetry between two branches of the time of flight spectrum. The asymmetry was measured on the NG-6 neutron beam at the NIST Center for Neutron Research (NCNR), and a subsequent measurement has been started on the higher flux NG-C beam. An overview of the method and systematic effects will be presented, including results from the NG-6 dataset.   

Limits on tensor currents from $^{8}$Li $\beta$ decay

Precision measurements of angular correlations from nuclear $\beta$ decay provide information on possible exotic couplings in the weak interaction. In the $\beta$ decay of $^{8}$Li, the delayed-$\alpha$ breakup of the $^{8}$Be$^{*}$ daughter provides enhanced sensitivity to possible tensor couplings. We report a limit on the ratio of the tensor to axial-vector coupling from the $^{8}$Li $\beta-\alpha-\nu$ correlation experiment performed using the $\beta$-decay Paul Trap (BPT) at Argonne National Laboratory. We will discuss our continued work on angular correlation measurements as well as new techniques being developed to overcome current experimental limitations.