A SEARCH FOR BARYON NUMBER VIOLATION BY TWO UNITS AT THE SUPER-KAMIOKANDE DETECTOR
This event is part of the PhD Final Oral Exams.
Edward Kearns, Robert Carey, Tulika Bose, Shyam Erramilli, Andrew Cohen
Baryon number appears to be a conserved quantity in the Standard Model of particle physics, though there are compelling theoretical reasons to believe it isn't. This thesis describes searches for processes that violate baryon number by two units with the Super-Kamiokande experiment. Two types of searches are performed. One is the simultaneous decay of two bound nucleons to pions (dinucleon decay), encompassing three different modes. The second is a search for a neutron transforming into an antineutron, or neutron-antineutron oscillation.
This thesis uses the full dataset across four Super-Kamiokande detector periods from April 1996 to March 2015, comprising 4972.4 livetime days (307 kiloton-years). Monte Carlo simulations of the signal processes and their background from atmospheric neutrino interactions are used to estimate signal efficiency, expected background, and their associated uncertainties. Both multivariate analyses and simple kinematical cuts are applied in these searches. This is the first search for dinucleon decay to pions in oxygen. The search for neutron-antineutron oscillation entails some important updates relative to a previous search at Super-Kamiokande, mainly increased exposure and an improved model of pion interactions.
In each mode investigated, no signal excess was observed, and all data are consistent with atmospheric neutrino background. In the absence of evidence for any signal process, lower lifetimes are set at the 90% confidence level. For dinucleon decay, these lower lifetimes are about two orders of magnitude more stringent than in a previous search in iron, while for neutron-antineutron oscillation the lower lifetime is comparable to previous searches.