Limits on Heavy Neutrino Mixing from the Beta Decay of ³⁸mK Confined in a Magneto-Optical Trap

This thesis details the search for mixing between electron neutrinos and possible heavy neutrinos using the positron decay of ³⁸mK produced at ISAC, TRIUMF's new radioactive beam facility. Using the novel technique of laser atom trapping to confine ³⁸mK, direct limits on such mixing in β-decay are determined for the mass range of 0.7 MeV/c² to 3.8 MeV/c². The results, for part of this mass range, provide the best direct limits for the mixing of heavy neutrinos with the electron neutrino in the literature to date. The Standard Model of electroweak physics has been able to describe a wide range of experimental results, but an unresolved question arises in accounting for lepton masses. The charged leptons (electrons, muons and taus) have masses which vary enormously from one to another (me = 0.511 MeV/c², mμ = 106 MeV/c², mτ = 1777 MeV/c²), while their accompanying neutrinos appear to have no (or very little) mass. If neutrinos have finite mass, then mixing could occur among the different neutrino species. Thus, evidence of mixing would indicate the existence of non-zero neutrino mass and of the need to modify or extend the Standard Model. Consequently, this is a highly active field of physics. Evidence of possible neutrino mass and oscillation has been recently observed by the detection of energetic neutrinos from the atmosphere. The TRINAT collaboration at TRIUMF has searched for evidence of physics beyond the Standard Model using a radioactive source of ³⁸mK atoms held in a neutral atom trap. The trap provides a source of several thousand atoms confined to a volume of less than a cubic millimeter, suspended in a vacuum vessel, in which the ³⁸mK undergoes radioactive decay with the emission of a positron and a neutrino. This results in the creation of a daughter nucleus, ³⁸Ar, with momentum equal to the vector sum of the momenta of the positron and the neutrino. The unperturbed nucleus then recoils from the trap and is detected in coincidence with the positron. Through the measurement of the positron's direction and energy along with the nucleus' direction and time of flight, the neutrino momentum is deduced. The emission of a heavy neutrino would change the momentum distribution of the system and thus alter the time of flight of the recoiling nucleus as a function of positron energy. The technique permits the reconstruction of the data event-by-event, which results in an increase in both the mass range and sensitivity of the search for the possible emission of heavy neutrinos. The data were reconstructed for the recoiling nucleus' time of flight and compared to detailed Monte Carlo simulations of the TRINAT detection system. A 2-dimensional maximum likelihood search in recoil time of flight and positron energy was used to place limits on the fraction of decays involving the emission of a heavy neutrino as a function of the mass of that neutrino.Non UBCUnreviewedGraduat