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Register a new userNeutron-Antineutron Oscillation Search using a 0.37 Megaton$cdot$Year Exposure of Super-Kamiokande
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As a baryon number violating process with $Delta B=2$, neutron-antineutron oscillation ($ntobar n$) provides a unique test of baryon number conservation. We have performed a search for $ntobar n$ oscillation with bound neutrons in Super-Kamiokande, with the full data set from its first four run periods, representing an exposure of 0.37~Mton-years. The search used a multivariate analysis trained on simulated $ntobar n$ events and atmospheric neutrino backgrounds and resulted in 11 candidate events with an expected background of 9.3 events. In the absence of statistically significant excess, we derived a lower limit on $bar n$ appearance lifetime in $^{16}$O nuclei of $3.6times{10}^{32}$ years and on the neutron-antineutron oscillation time of $tau_{ntobar n} > 4.7times10^{8}$~s at 90% C.L..
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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.
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.
We have searched for proton decay via $p rightarrow u K^{+}$ using Super-Kamiokande data from April 1996 to February 2013, 260 kiloton$cdot$year exposure in total. No evidence for this proton decay mode is found. A lower limit of the proton lifetime is set to $5.9 times 10^{33}$ years at 90% confidence level.
A search for neutron-antineutron ($n-bar{n}$) oscillation was undertaken in Super-Kamiokande using the 1489 live-day or $2.45 times 10^{34}$ neutron-year exposure data. This process violates both baryon and baryon minus lepton numbers by an absolute value of two units and is predicted by a large class of hypothetical models where the seesaw mechanism is incorporated to explain the observed tiny neutrino masses and the matter-antimatter asymmetry in the Universe. No evidence for $n-bar{n}$ oscillation was found, the lower limit of the lifetime for neutrons bound in ${}^{16}$O, in an analysis that included all of the significant sources of experimental uncertainties, was determined to be $1.9 times 10^{32}$~years at the 90% confidence level. The corresponding lower limit for the oscillation time of free neutrons was calculated to be $2.7 times 10^8$~s using a theoretical value of the nuclear suppression factor of $0.517 times 10^{23}$~s$^{-1}$ and its uncertainty.
We present a search for differences in the oscillations of antineutrinos and neutrinos in the Super-Kamiokande -I, -II, and -III atmospheric neutrino sample. Under a two-flavor disappearance model with separate mixing parameters between neutrinos and antineutrinos, we find no evidence for a difference in oscillation parameters. Best fit antineutrino mixing is found to be at (dm2bar, sin2 2 thetabar) = (2.0x10^-3 eV^2, 1.0) and is consistent with the overall Super-K measurement.
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