Neutron Decay Correlations In The Nab Experiment

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.