Search results for nucleon decays $p\to e^{+}X$, $p\to {\mu }^{+}X$, $n\to \nu \gamma$ (where $X$ is an invisible, massless particle) as well as dinucleon decays $np\to e^{+}\nu$, $np\to {\mu }^{+}\nu$, and $np\to {\tau }^{+}\nu$ in the Super-Kamiokande experiment are presented. Using single-ring data from an exposure of $273.4\mathrm{kton}·\mathrm{yr}$, a search for these decays yields a result consistent with no signal. Accordingly, lower limits on the partial lifetimes of ${\tau }_{p\to e^{+}X}>7.9\times {10}^{32}\mathrm{yr}$, ${\tau }_{p\to {\mu }^{+}X}>4.1\times {10}^{32}\mathrm{yr}$, ${\tau }_{n\to \nu \gamma }>5.5\times {10}^{32}\mathrm{yr}$, ${\tau }_{np\to e^{+}\nu }>2.6\times {10}^{32}\mathrm{yr}$, ${\tau }_{np\to {\mu }^{+}\nu }>2.2\times {10}^{32}\mathrm{yr}$, and ${\tau }_{np\to {\tau }^{+}\nu }>2.9\times {10}^{31}\mathrm{yr}$ at a 90% confidence level are obtained. Some of these searches are novel.

• Received 5 August 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.121803