Grant: Subatomic Physics Envelope - Project

We use laser trapping and cooling techniques to capture radioactive atoms and precisely test the Standard Model (S.M.) of the weak interaction. Confined in a 1 mm-sized cloud, the nuclei undergo beta decay, producing three products: a positron, a neutrino, and a recoiling daughter nucleus. The daughter nucleus has only a few hundred eV of energy and would stop in a nm of material, but it freely escapes the trap. By detecting it in coincidence with the positron, we can measure the momentum and hence the angular distribution of neutrinos more directly than in previous experiments. We have set the best limits on scalar interactions coupling to the first generation of particles by measuring the beta-neutrino correlation in the Fermi decay of a spin-0 nucleus. An approved proposal at high priority will extend these measurements to lower beta energy to compete directly with the Fierz scalar-vector interference term constraints from the Q-value dependence of many 0+ to 0+ Ft experiments. Our planned methods would halve their error to 0.001, providing meaningful sensitivity to one theoretical source of semileptonic interactions (left-right SUSY fermion mixing) that is difficult to constrain in high-energy experiments. We have learned to spin-polarize other nuclei and measure the polarization of the decaying species by atomic methods independent of the nuclear decays. When combined with beta-recoil coincidences, this capability allows us to measure novel correlations. We have published a measurement of the neutrino asymmetry of 37K with 3% errors. A new proposal for spin-correlation measurements to 0.1 to 0.3% level by Dan Melconian of Texas A&M has been accepted by the TRIUMF EEC at high priority. We have developed techniques for the above experiments by doing other experiments suitable for students. Two M.Sc.'s were done on the spin asymmetry of daughter nuclei in 80Rb decay, and we have submitted for publication limits on tensor interactions. A Ph.D. student is searching for exotic massive particles in two-body nuclear isomer decay, developing a spectrometer to precisely measure the nuclear daughter momenta.