Developments for the $^6$He $\beta-\bar{\nu}$ angular correlation experiment

Abstract

This thesis describes developments toward the measurement of the angular correlation between the beta and the antineutrino in the beta decay of $^6$He. This decay is a pure Gamow-Teller decay which is described in the Standard Model as a purely axial vector weak interaction. The angular correlation is characterized by the parameter $a_{\beta\nu}=-1/3$ in the Standard Model. Any deviation from this value would be evidence for tensor components in the weak interaction and would constitute new physics. A new method will be used to measure the parameter $a_{\beta\nu}$ from $^6$He decays, featuring a magneto-optical trap that will measure the beta particle in coincidence with the recoiling $^6$Li daughter ion. This neutral atom trapping scheme provides cold, tightly confined atoms which will reduce systematic uncertainties related to the initial position of the decay. By knowing the initial position of the decay and measuring the time of flight of the recoiling $^6$Li daughter ion in coincidence with the beta, the angular correlation between the beta and the antineutrino can be deduced. We aim to measure $a_{\beta\nu}$ first to the level of $1\%$, and eventually to the $0.1\%$ level, which would represent an order of magnitude improvement in precision over past experiments. Towards this goal, we have designed, built, and successfully tested a liquid lithium target to provide $>2\times10^{10}$ $^6$He atoms/s to a low-background environment, which is the most intense source of $^6$He presently available. This allowed for an additional measurement of the $^6$He half-life ($806.89\pm0.11_{stat}{}^{+0.23}_{−-0.19}{}_{syst}$~ms) to be made with unprecedented precision, resolving discrepancies in past measurements. We have also tested our trapping and detection apparatus and have begun to record preliminary coincidence events.