Grant: Subatomic Physics Envelope - Project

Nuclear beta decay correlation experiments helped establish the nature of the weak interaction, a theory with spin-1 bosons coupling only to left-handed leptons. Such measurements can still assist particle physics by determining 1st-generation lepton-quark couplings of possible new particles, but must reach excellent accuracy of 0.1%. Using atom trap technology, we are poised to complete two experiments reaching this accuracy. 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 very little kinetic 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 angular distribution of neutrinos. We have learned to spin-polarize nuclei and measure the polarization of the decaying species by atomic methods independent of the nuclear decays. A new proposal to improve our spin-correlation measurements (like the asymmetry of decay of betas and neutrinos) to 0.1 to 0.3% level by Dan Melconian of Texas A&M has been accepted by the TRIUMF EEC at high priority (two Texas A&M grad students). We are completing a larger vacuum chamber with larger beta detector solid angle, complete angular acceptance for nuclear recoils, and a detector for shakeoff atomic electrons to make time-of-flight singles measurements on the nuclear recoils and to eliminate few percent backgrounds on beta singles measurements. We have improved our ability to turn off the trapping magnetic fields quickly by implementing a new type of trap (UBC grad student). We also would improve our 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. The complete angular acceptance for recoils we have developed will minimize key systematic errors in our EEC-approved upgrade of this experiment, with a goal of reaching 0.1% accuracy. We determine systematic errors for this experiment from statistics-limited kinematic observables that are independent of the angular correlation.