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Searches for a nucleon decay into a charged anti-lepton (e^+ or {mu}^+) plus a light meson ({pi}^0, {pi}^-, {eta}, {rho}^0, {rho}^-, {omega}) were performed using the Super-Kamiokande I and II data. Twelve nucleon decay modes were searched for. The total exposure is 140.9 kiloton cdot years, which includes a 91.7 kiloton cdot year exposure (1489.2 live days) of Super-Kamiokande-I and a 49.2 kiloton cdot year exposure (798.6 live days) of Super-Kamiokande-II. The number of candidate events in the data was consistent with the atmospheric neutrino background expectation. No significant evidence for a nucleon decay was observed in the data. Thus, lower limits on the nucleon partial lifetime at 90% confidence level were obtained. The limits range from 3.6 times 10^31 to 8.2 times 10^33 years, depending on the decay modes.
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We have searched for proton into muon plus neutral kaon using data from a 91.7 kiloton-year exposure of Super-Kamiokande-I, a 49.2 kiloton-year exposure of Super-Kamiokande-II, and a 31.9 kiloton-year exposure of Super-Kamiokande-III. The number of candidate events in the data was consistent with the atmospheric neutrino background expectation and no evidence for proton decay in this mode was found. We set a partial lifetime lower limit of 1.6x10^33 years at the 90% confidence level.
We have searched for proton decays into a charged antilepton ($e^+$, $mu^+$) plus a meson ($eta$, $rho^0$, $omega$) and neutron decays into a charged antilepton ($e^+$, $mu^+$) plus a meson ($pi^-$, $rho^-$) using Super-Kamiokande (SK) I-IV data, corresponding to 0.316 megaton$cdot$years of exposure. This measurement updates the previous published result by using 2.26 times more data and improved analysis methods. No significant evidence for nucleon decay has been observed and lower limits on the partial lifetime of the nucleon are obtained. The limits range from 3$times$10$^{31}$ to 1$times$10$^{34}$ years at 90% confidence level, depending on the decay mode.
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$(pp) rightarrow$ $^{14}$C$pi^{+}pi^{+}$, $^{16}$O$(pn) rightarrow$ $^{14}$N$pi^{+}pi^{0}$, and $^{16}$O$(nn) rightarrow$ $^{14}$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: $tau_{pprightarrowpi^{+}pi^{+}} > 7.22 times 10^{31}$ years, $tau_{pnrightarrowpi^{+}pi^{0}} > 1.70 times 10^{32}$ years, and $tau_{nnrightarrowpi^{0}pi^{0}} > 4.04 times 10^{32}$ years. The lower limits on each mode are about two orders of magnitude better than previous limits from searches for dinucleon decay in iron.
A search for proton decay into three charged leptons has been performed by using 0.37$,$Mton$cdot$years of data collected in Super-Kamiokande. All possible combinations of electrons, muons and their anti-particles consistent with charge conservation were considered as decay modes. No significant excess of events has been found over the background, and lower limits on the proton lifetime divided by the branching ratio have been obtained. The limits range between $9.2times10^{33}$ to $3.4times10^{34}$ years at 90$,$% confidence level, improving by more than an order of magnitude upon limits from previous experiments. A first limit has been set for the $prightarrowmu^-e^+e^+$ mode.
Search results for nucleon decays $p rightarrow e^+X$, $p rightarrow mu^+X$, $n rightarrow ugamma$ (where $X$ is an invisible, massless particle) as well as dinucleon decays $np rightarrow e^+ u$, $np rightarrow mu^+ u$ and $np rightarrow tau^+ u$ in the Super-Kamiokande experiment are presented. Using single-ring data from an exposure of 273.4 kton $cdot$ years, 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, $tau_{p rightarrow mu^+X} > 4.1 times 10^{32}$ years, $tau_{n rightarrow ugamma} > 5.5 times 10^{32}$ years, $tau_{np rightarrow e^+ u} > 2.6 times 10^{32}$ years, $tau_{np rightarrow mu^+ u} > 2.2 times 10^{32}$ years and $tau_{np rightarrow tau^+ u} > 2.9 times 10^{31}$ years at a $90 % $ confidence level are obtained. Some of these searches are novel.
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