TRIUMF neutral atom trap for beta decay studies

点击翻译按钮获取中文摘要

Nuclear beta decay correlation experiments helped establish the nature of the weak interaction, a theory with massive bosons in addition to the photon coupling only to `left-handed' neutrinos. 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% with high significance. Using atom trap technology, we are poised to complete two sets of experiments reaching the 0.1% accuracy needed. 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 the neutrino. We have learned to spin-polarize 37K nuclei by optical pumping, achieving 99.1+-0.1%. We atomically probe the 37K atoms as their decays are measured. We consider this a major breakthrough that enables several new spin-polarized experiments, including the asymmetry of recoils with respect to the nuclear spin. We also would improve our best direct limits on general 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 are developing will minimize key systematic errors in our TRIUMF EEC-approved upgrade of this experiment, with a goal of reaching 0.1% accuracy. We would determine systematic errors for this experiment from statistics-limited kinematic observables insensitive to the angular correlation. This would be complementary to the energy dependence of superallowed Ft measurements, and to indirect constraints from the pi to e nu branching ratio. A new possible source of time-reversal violation (TRV) produces a correlation between the momenta of the beta, neutrino, and the radiative gamma sometimes emitted. Such experiments have been done in radiative kaon decay, but not in systems involving light quarks. The TRIUMF EEC has approved our proposal to measure the radiative beta decay TRV asymmetry, which we plan to measure at better than 0.01 accuracy, an order of magnitude improvement on indirect limits. Radiative beta decay TRV asymmetries, because they involve the gamma-ray, can provide access to interesting weakly coupled neutral current physics even in a charge-changing decay. Precision multidisciplinary experiments combining atomic and nuclear techniques provide research opportunities for undergrad and grad students. Typically the undergrad research is on self-contained atomic physics tools needed for the beta decay measurements, while the grad students start with masters-level instrumentation leading to a full experiment and its analysis and interpretation.