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Register a new userSearch for Differences in Oscillation Parameters for Atmospheric Neutrinos and Antineutrinos at Super-Kamiokande
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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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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.
GUT monopoles captured by the Suns gravitation are expected to catalyze proton decays via the Callan-Rubakov process. In this scenario, protons, which initially decay into pions, will ultimately produce u_{e}, u_{mu} and bar{ u}_{mu}. After undergoing neutrino oscillation, all neutrino species appear when they arrive at the Earth, and can be detected by a 50,000 metric ton water Cherenkov detector, Super-Kamiokande (SK). A search for low energy neutrinos in the electron total energy range from 19 to 55 MeV was carried out with SK and gives a monopole flux limit of F_M(sigma_0/1 mb) < 6.3 times 10^{-24} (beta_M/10^{-3})^2 cm^{-2} s^{-1} sr^{-1} at 90% C.L., where beta_M is the monopole velocity in units of the speed of light and sigma_0 is the catalysis cross section at beta_M=1. The obtained limit is more than eight orders of magnitude more stringent than the current best cosmic-ray supermassive monopole flux limit, F_M < 1 times 10^{-15} cm^{-2} s^{-1} sr^{-1} for beta_M < 10^{-3} and also two orders of magnitude lower than the result of the Kamiokande experiment, which used a similar detection method.
The NOvA experiment has made a $4.4sigma$-significant observation of $bar u_{e}$ appearance in a 2 GeV $bar u_{mu}$ beam at a distance of 810 km. Using $12.33times10^{20}$ protons on target delivered to the Fermilab NuMI neutrino beamline, the experiment recorded 27 $bar u_{mu} rightarrow bar u_{e}$ candidates with a background of 10.3 and 102 $bar u_{mu} rightarrow bar u_{mu}$ candidates. This new antineutrino data is combined with neutrino data to measure the oscillation parameters $|Delta m^2_{32}| = 2.48^{+0.11}_{-0.06}times10^{-3}$ eV$^2/c^4$, $sin^2 theta_{23} = 0.56^{+0.04}_{-0.03}$ in the normal neutrino mass hierarchy and upper octant and excludes most values near $delta_{rm CP}=pi/2$ for the inverted mass hierarchy by more than 3$sigma$. The data favor the normal neutrino mass hierarchy by 1.9$sigma$ and $theta_{23}$ values in the upper octant by 1.6$sigma$.
We present limits on sterile neutrino mixing using 4,438 live-days of atmospheric neutrino data from the Super-Kamiokande experiment. We search for fast oscillations driven by an eV$^2$-scale mass splitting and for oscillations into sterile neutrinos instead of tau neutrinos at the atmospheric mass splitting. When performing both these searches we assume that the sterile mass splitting is large, allowing $sin^2(Delta m^2 L/4E)$ to be approximated as $0.5$, and we assume that there is no mixing between electron neutrinos and sterile neutrinos ($|U_{e4}|^2 = 0$). No evidence of sterile oscillations is seen and we limit $|U_{mu4}|^2$ to less than 0.041 and $|U_{tau4}|^2$ to less than 0.18 for $Delta m^2 > 0.8$ eV$^2$ at the 90% C.L. in a 3+1 framework. The approximations that can be made with atmospheric neutrinos allow these limits to be easily applied to 3+N models, and we provide our results in a generic format to allow comparisons with other sterile neutrino models.
A new event reconstruction algorithm based on a maximum likelihood method has been developed for Super-Kamiokande. Its improved kinematic and particle identification capabilities enable the analysis of atmospheric neutrino data in a detector volume 32% larger than previous analyses and increases sensitivity to the neutrino mass hierarchy. Analysis of a 253.9 kton-year exposure of the Super-Kamiokande IV atmospheric neutrino data has yielded a weak preference for the normal hierarchy, disfavoring the inverted hierarchy at 74% assuming oscillations at the best fit of the analysis.
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