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Register a new userReview of Atmospheric Neutrino Results from Super-Kamiokande
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Volodymyr Takhistov
Publication date
2020
fields
Physics
and research's language is
English
Authors
Volodymyr Takhistov
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While neutrino physics enters precision era, several important unknowns remain. Atmospheric neutrinos allow to simultaneously test key oscillation parameters, with Super-Kamiokande experiment playing a central role. We discuss results from atmospheric neutrino oscillation analysis of the full dataset from Super-Kamiokande I-IV phases. Further, we discuss tests of non-standard neutrino interactions with atmospheric neutrinos in Super-Kamiokande.
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Recent results from a 282 kiloton-year exposure of the Super-Kamiokande detector to atmospheric neutrinos are presented. The data when fit both by themselves and in conjunction with constraints from the T2K and reactor neutrino experiments show a weak, though insignificant, preference for the normal mass hierarchy at the level of ~1 sigma. Searches for evidence of oscillations into a sterile neutrino have resulted in limits on the parameters governing their mixing, |U_mu4}|^2 <0.041 and |U_tau4|^2 < 0.18 at 90% C.L. A similar search for an indication of Lorentz-invariance violating oscillations has yielded limits three to seven orders of magnitude more stringent than existing measurements. Additionally, analyses searching for an excess of neutrinos in the atmospheric data produced from the annihilation of dark matter particles in the galaxy and sun have placed tight limits on the cross sections governing their annihilation and scattering.
The results of the third phase of the Super-Kamiokande solar neutrino measurement are presented and compared to the first and second phase results. With improved detector calibrations, a full detector simulation, and improved analysis methods, the systematic uncertainty on the total neutrino flux is estimated to be ?2.1%, which is about two thirds of the systematic uncertainty for the first phase of Super-Kamiokande. The observed 8B solar flux in the 5.0 to 20 MeV total electron energy region is 2.32+/-0.04 (stat.)+/-0.05 (sys.) *10^6 cm^-2sec^-1, in agreement with previous measurements. A combined oscillation analysis is carried out using SK-I, II, and III data, and the results are also combined with the results of other solar neutrino experiments. The best-fit oscillation parameters are obtained to be sin^2 {theta}12 = 0.30+0.02-0.01(tan^2 {theta}12 = 0.42+0.04 -0.02) and {Delta}m2_21 = 6.2+1.1-1.9 *10^-5eV^2. Combined with KamLAND results, the best-fit oscillation parameters are found to be sin^2 {theta}12 = 0.31+/-0.01(tan^2 {theta}12 = 0.44+/-0.03) and {Delta}m2_21 = 7.6?0.2*10^-5eV^2 . The 8B neutrino flux obtained from global solar neutrino experiments is 5.3+/-0.2(stat.+sys.)*10^6cm^-2s^-1, while the 8B flux becomes 5.1+/-0.1(stat.+sys.)*10^6cm^-2s^-1 by adding KamLAND result. In a three-flavor analysis combining all solar neutrino experiments, the upper limit of sin^2 {theta}13 is 0.060 at 95% C.L.. After combination with KamLAND results, the upper limit of sin^2 {theta}13 is found to be 0.059 at 95% C.L..
A comprehensive study on the atmospheric neutrino flux in the energy region from sub-GeV up to several TeV using the Super-Kamiokande water Cherenkov detector is presented in this paper. The energy and azimuthal spectra of the atmospheric ${ u}_e+{bar{ u}}_e$ and ${ u}_{mu}+{bar{ u}}_{mu}$ fluxes are measured. The energy spectra are obtained using an iterative unfolding method by combining various event topologies with differing energy responses. The azimuthal spectra depending on energy and zenith angle, and their modulation by geomagnetic effects, are also studied. A predicted east-west asymmetry is observed in both the ${ u}_e$ and ${ u}_{mu}$ samples at 8.0 {sigma} and 6.0 {sigma} significance, respectively, and an indication that the asymmetry dipole angle changes depending on the zenith angle was seen at the 2.2 {sigma} level. The measured energy and azimuthal spectra are consistent with the current flux models within the estimated systematic uncertainties. A study of the long-term correlation between the atmospheric neutrino flux and the solar magnetic activity cycle is also performed, and a weak indication of a correlation was seen at the 1.1 {sigma} level, using SK I-IV data spanning a 20 year period. For particularly strong solar activity periods known as Forbush decreases, no theoretical prediction is available, but a deviation below the typical neutrino event rate is seen at the 2.4 {sigma} level.
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.
Using 5,326 days of atmospheric neutrino data, a search for atmospheric tau neutrino appearance has been performed in the Super-Kamiokande experiment. Super-Kamiokande measures the tau normalization to be 1.47$pm$0.32 under the assumption of normal neutrino hierarchy, relative to the expectation of unity with neutrino oscillation. The result excludes the hypothesis of no-tau-appearance with a significance level of 4.6$sigma$. The inclusive charged-current tau neutrino cross section averaged by the tau neutrino flux at Super-Kamiokande is measured to be $(0.94pm0.20)times 10^{-38}$ cm$^{2}$. The measurement is consistent with the Standard Model prediction, agreeing to within 1.5$sigma$.
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