UC Irvine

Various theoretical considerations suggest that baryon number should be violated. Nucleon decay, which typically appears within the context of unified theories, would provide a definitive signature of baryon number violation. In this dissertation, we report on the search results for $p \rightarrow e^+X$, $p \rightarrow \mu^+$ (where $X$ is an invisible, massless particle), $n \rightarrow \nu\gamma$, $p \rightarrow e^+\nu\nu$, $p \rightarrow \mu^+\nu\nu$, $np \rightarrow e^+\nu$, $np \rightarrow \mu^+\nu$ and

$np \rightarrow \tau^+\nu$ nucleon and dinucleon decays at the Super-Kamiokande experiment. Some of these searches are novel. Using data from a combined exposure of 273.4 kton$\cdot$years and a $\chi^2$ spectral fitting technique, a search for these decays yields a result consistent with no signal. Accordingly, lower limits on the partial lifetimes of $\tau_{p \rightarrow e^+ X} > 7.9 \times 10^{32}$ years, $\tau_{n \rightarrow \nu\gamma} > 5.5 \times 10^{32}$ years, $\tau_{p \rightarrow \mu^+ X} > 4.1 \times 10^{32}$ years, $\tau_{p \rightarrow e^+\nu\nu} > 1.7 \times 10^{32}$ years, $\tau_{p \rightarrow \mu^+\nu\nu} > 2.2 \times 10^{32}$ years, $\tau_{np \rightarrow e^+\nu} > 2.6 \times 10^{32}$ years, $\tau_{np \rightarrow \mu^+\nu} > 2.0 \times 10^{32}$ years and

$\tau_{np \rightarrow \tau^+\nu} > 3.0 \times 10^{31}$ years at a 90\% confidence level are obtained. These results provide stringent test of new physics and also limit the parameter space of models that allow for such processes.