Search for dinucleon decay into pions at Super-Kamiokande

A search for dinucleon decay into pions with the Super-Kamiokande detector has been performed with an exposure of 282.1 kiloton-years. Dinucleon decay is a process that violates baryon number by two units. We present the first search for dinucleon decay to pions in a large water Cherenkov detector. The modes ${}^{16}$O$(pp)\to$ ${}^{14}$C${\pi }^{+}{\pi }^{+}$, ${}^{16}$O$(pn)\to$ ${}^{14}$N${\pi }^{+}{\pi }^0$, and ${}^{16}$O$(nn)\to$ ${}^{14}$O${\pi }^0{\pi }^0$ are investigated. No significant excess in the Super-Kamiokande data has been found, so a lower limit on the lifetime of the process per oxygen nucleus is determined. These limits are: ${\tau }_{pp\to {\pi }^{+}{\pi }^{+}}>7.22\times {10}^{31}$ years, ${\tau }_{pn\to {\pi }^{+}{\pi }^0}>1.70\times {10}^{32}$ years, and ${\tau }_{nn\to {\pi }^0{\pi }^0}>4.04\times {10}^{32}$ years. The lower limits on each mode are about two orders of magnitude better than previous limits from searches for dinucleon decay in iron.