Fundamental symmetry tests with optically trapped radioactive neutral atoms

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We use laser trapping and cooling techniques to capture radioactive atoms and precisely test the Standard Model 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 daughter nucleus. The daughter moves slowly 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.   The known electroweak interaction is mediated by spin-1 vector bosons, the W+, W-, and Z0. These only produce left-handed neutrinos: parity (mirror symmetry) is maximally violated. We test these model features.   We have now published the best limits on scalar interactions coupling to the first generation of particles by measuring the beta-neutrino correlation in the pure Fermi decay of a spin-0 nucleus. We propose to improve these measurements by a factor of three. We published a missing-mass search for heavy neutrinos mixing with the electron neutrino, directly constraining admixture levels of interest to astrophysics.   We are learning to spin-polarize other nuclei and measure their polarization by atomic methods independent of their decays. When combined with beta-recoil coincidences, we can measure novel correlations. We have measured the neutrino asymmetry of 37K to 3% accuracy, which we intend to improve to 0.1%   The spin asymmetry of the direction of the daughter nuclei vanishes in pure Gamow-Teller decay. We can now uniquely measure this correlation with the atom trap, and since it vanishes many polarization systematics become unimportant. Our goal is to reach 0.1% accuracy to test hints of a nonzero tensor interaction reported by the PIBETA collaboration in radiative pion decays. This is the project of two M.Sc. students. We will soon graduate our 2nd Ph.D., to be followed by a 3rd, graduated an M.Sc. on a new type of trap, and average one undergrad at a time.