Search For Invisible Nucleon Decay in SNO+ With Improved Sensitivity

Abstract

The water phase of the SNO+ (Sudbury Neutrino Observatory) experiment has completed after the collection of ~305 days of total livetime worth of data, divided into two main sets. The effective concentration of U-238 and Th-232 in the water was determined using in situ analysis to be, for the second set of data, gU/gH2O: (3.64 +/- 0.74_(stat) + 1.28_(syst) - 0.99_(syst) x 10^-15 and gTh/gH2O: (3.08 +/- 3.22_(stat) + 1.60_(syst) - 5.00_(syst) ) x 10^-16. The water data can also been used to search for invisible" modes of nucleon decay, and a previous SNO+ publication set world-leading limits on relevant nucleon and dinucleon decay modes except neutron and dineutron decay. This thesis presents a re-analysis of the invisible" neutron decay mode of that first set of data, which reoptimizes the fiducial volume and develops an improved treatment of the dominant energy systematic uncertainties. This leads to a world-leading lower limit on the ``invisible" neutron decay lifetime of 6.91 x 10^29 years. A sensitivity analysis shows that including the second set of data that has lower backgrounds is expected to yield a lower limit on the neutron decay lifetime of 1.57 x 10^30 years, further improving the result.