Super-Kamiokande Atmospheric Neutrino Data Research Articles

Atmospheric neutrinos and proton decay in Super-Kamiokande and Hyper-Kamiokande

Super-Kamiokande is a 50 kton water Cherenkov detector, which has been in operation since 1996. Super-Kamiokande atmospheric neutrino data have a preference for the normal neutrino mass hierarchy (Δχ2=χNH2−χIH2=−4.3) when the constraints from reactor neutrino experiments are included. The search for tau neutrino appearance from neutrino oscillations has resulted in a 4.6σ exclusion of the hypothesis of no tau appearance. Hyper-Kamiokande is a proposed next-generation water Cherenkov detector, which will be a natural extension of Super-Kamiokande. The proposed experiment will have two cylindrical tanks containing 520 kton of water in total. Hyper-K will search for CP violation using the neutrino beam from J-PARC, and will have a broad physics program including studies of atmospheric neutrinos, supernova burst neutrinos, geo-neutrinos and searches for proton decay.

Test of Lorentz invariance with atmospheric neutrinos

A search for neutrino oscillations induced by Lorentz violation has been performed using 4,438 live-days of Super-Kamiokande atmospheric neutrino data. The Lorentz violation is included in addition to standard three-flavor oscillations using the non-perturbative Standard Model Extension (SME), allowing the use of the full range of neutrino path lengths, ranging from 15 to 12,800 km, and energies ranging from 100 MeV to more than 100 TeV in the search. No evidence of Lorentz violation was observed, so limits are set on the renormalizable isotropic SME coefficients in the $e\mu$, $\mu\tau$, and $e\tau$ sectors, improving the existing limits by up to seven orders of magnitude and setting limits for the first time in the neutrino $\mu\tau$ sector of the SME.

Evidence for the Appearance of Atmospheric Tau Neutrinos in Super-Kamiokande

Super-Kamiokande atmospheric neutrino data were fit with an unbinned maximum likelihood method to search for the appearance of tau leptons resulting from the interactions of oscillation-generated tau neutrinos in the detector. Relative to the expectation of unity, the tau normalization is found to be 1.42 ± 0.35(stat)(-0.12)(+0.14)(syst) excluding the no-tau-appearance hypothesis, for which the normalization would be zero, at the 3.8σ level. We estimate that 180.1 ± 44.3(stat)(-15.2)(+17.8) (syst) tau leptons were produced in the 22.5 kton fiducial volume of the detector by tau neutrinos during the 2806 day running period. In future analyses, this large sample of selected tau events will allow the study of charged current tau neutrino interaction physics with oscillation produced tau neutrinos.

High Energy Neutrinos in Super-Kamiokande

The new analysis of Super-Kamiokande atmospheric neutrino data, based on a subsample of higher precision data is reported. The plot of the muon neutrino survival probability as a function of the ratio of the distance over neutrino energy displays a dip just where the first oscillation maximum is expected from the standard analysis. As a result the parameters of the transitions νμ↔ντ are better constrained: 1.9×10−3<Δm2<3.0×10−3 eV2 and sin22θ>0.90 at 90% C.L. A signal of WIMP annihilations in the Sun, the core of the Earth or the Galactic Center has been searched in a sample of upward through-going muons, produced by neutrino interactions outside of detector. No statistically significant signal has been observed. Upper limits of the muon fluxes are compared with results from other neutrino detectors. Also upper limits on cross section for the WIMP nucleon interaction are derived and compared with corresponding results from direct searches of dark matter particles.

Updated analysis of atmospheric neutrino data

In the context of the latest (79.5 kTy) Super-Kamiokande atmospheric neutrino data, we present an updated analysis of the mass and mixing parameters relevant for two-, three-, and four-family neutrino mixing. Oscillations in the ν μ→ν τ channel clearly emerge as the dominant mechanism responsible for the atmospheric ν μ anomaly, with no significant indication for subdominant mixing with electron or sterile neutrinos. Upper bounds on subdominant mixing angles are also discussed.

Author Index

Superkamiokande atmospheric neutrino data for 990 days are analyzed in the framework of four neutrinos without imposing constraints of Big Bang Nucleosynthesis. It is shown that the wide range of the oscillation parameters is allowed at 90% confidence level (0.1≲|Us1|2+|Us2|2⩽1).

Three generation neutrino oscillation parameters after SNO

We examine the solar neutrino problem in the context of the realistic three neutrino mixing scenario including the SNO charged current (CC) rate. The two independent mass squared differences $\ensuremath{\Delta}{m}_{21}^{2}$ and $\ensuremath{\Delta}{m}_{31}^{2}\ensuremath{\approx}\ensuremath{\Delta}{m}_{32}^{2}$ are taken to be in the solar and atmospheric ranges, respectively. We incorporate the constraints on $\ensuremath{\Delta}{m}_{31}^{2}$ as obtained by the SuperKamiokande atmospheric neutrino data and determine the allowed values of $\ensuremath{\Delta}{m}_{21}^{2},$ ${\ensuremath{\theta}}_{12}$ and ${\ensuremath{\theta}}_{13}$ from a combined analysis of solar and CHOOZ data. Our aim is to probe the changes in the values of the mass and mixing parameters with the inclusion of the SNO data as well as the changes in the two-generation parameter region obtained from the solar neutrino analysis with the inclusion of the third generation. We find that the inclusion of the SNO CC rate in the combined $\mathrm{solar}+\mathrm{CHOOZ}$ analysis puts a more restrictive bound on ${\ensuremath{\theta}}_{13}.$ Since the allowed values of ${\ensuremath{\theta}}_{13}$ are constrained to very small values by the CHOOZ experiment there is no qualitative change over the two generation allowed regions in the $\ensuremath{\Delta}{m}_{21}^{2}\ensuremath{-}{\mathrm{tan}}^{2}{\ensuremath{\theta}}_{12}$ plane. The best fit comes in the LMA region and no allowed area is obtained in the SMA region at the $3\ensuremath{\sigma}$ level from combined solar and CHOOZ analysis.

A three generation oscillation analysis of the Super-Kamiokande atmospheric neutrino data beyond one mass scale dominance approximation

In this paper we do a three-generation oscillation analysis of the latest (1144 days) Super-Kamiokande (SK) atmospheric neutrino data going beyond the one mass scale dominance (OMSD) approximation. We fix Δ 12= Δ 13 ( Δ LSND) in the range eV 2 as allowed by the results from LSND and other accelerator and reactor experiments on neutrino oscillation and keep Δ 23 ( Δ ATM) and the three mixing angles as free parameters. We incorporate the matter effects, indicate some new allowed regions with small Δ 23 (<10 −4 eV 2) and sin 22 θ 23 close to 0 and discuss the differences with the two-generation and OMSD pictures. In our scenario, the oscillation probabilities for the accelerator and reactor neutrinos involve only two of the mixing angles θ 12 and θ 13 and one mass scale. But the atmospheric neutrino oscillation is in general governed by both mass scales and all the three mixing angles. The higher mass scale gives rise to Δm 2 independent average oscillations for atmospheric neutrinos and does not enter the χ 2 analysis as an independent parameter. The Δ 23 and the three mixing angles on the other hand appear as independent parameters in the χ 2 analysis and the best-fit values of these are determined from an analysis of (a) the SK data, (b) the SK and CHOOZ data. The allowed values of the mixing angles θ 12 and θ 13 from the above analysis are compared with the constraints from all accelerator and reactor experiments including the latest results from LSND and KARMEN2. Implications for future long baseline experiments are discussed.

CP-VIOLATION IN NEUTRINO OSCILLATIONS AND L/E FLATNESS OF THE E-LIKE EVENT RATIO AT SUPER-KAMIOKANDE

We show that if the presently observed L/E-flatness of the electron-like event ratio in the Super-Kamiokande atmospheric neutrino data is confirmed, then the indicated ratio must be unity. Further, it is found that once CP is violated the exact L/E flatness implies: (a) the CP-violating phase, in the standard parametrization, is narrowed down to two possibilities ±π/2, and (b) the mixing between the second and the third generations must be maximal. With these results at hand, we argue that a dedicated study of the L/E-flatness of the electron-like event ratio by Super-Kamiokande can serve as an initial investigatory probe of CP violation in the neutrino sector. The assumptions under which these results hold are explicitly stated.

Super-Kamiokande atmospheric neutrinos: Status of subdominant oscillations

In the context of the recent (79.5 kTy) Super-Kamiokande atmospheric neutrino data, we concisely review the status of muonic-tauonic flavor oscillations and of the subdominant electron or sterile neutrino mixing, in schemes with three or four families and one dominant mass scale. In the three-family case, where we include the full CHOOZ spectral data, we also show, through a specific example, that ``maximal'' violations of the one-dominant mass scale approximation are not ruled out yet.

Analysis of the Superkamiokande atmospheric neutrino data in the framework of four neutrino mixings

Superkamiokande atmospheric neutrino data for 990 days are analyzed in the framework of four neutrinos without imposing constraints of Big Bang Nucleosynthesis. It is shown that the wide range of the oscillation parameters is allowed at 90% confidence level (0.1≲|Us1|2+|Us2|2⩽1).

Is neutrino decay really ruled out as a solution to the atmospheric neutrino problem from Super-Kamiokande data?

In this paper, we perform a detailed χ 2-analysis of the 848 days of Super-Kamiokande (SK) atmospheric neutrino data under the assumptions of ν μ –ν τ oscillation and neutrino decay. For the latter, we take the most general case of neutrinos with non-zero mixing and consider the possibilities of the unstable component in ν μ decaying to a state with which it mixes (scenario (a)) and to a sterile state with which it does not mix (scenario (b)). In the first case, Δm 2 (mass squared difference between the two mass states which mix) has to be >0.1 eV 2 from constraints on K decays, whereas for the second case, Δm 2 can be unconstrained. For case (a), Δm 2 does not enter the χ 2-analysis, while in case (b), it enters the χ 2-analysis as an independent parameter. In scenario (a), there is Δm 2 averaged oscillation in addition to decay, and this gets ruled out at 100.0% CL by the latest SK data. Scenario (b) on the other hand, gives a reasonably good fit to the data for Δm 2 ∼0.001 eV 2.

On the interpretation of the atmospheric neutrino data in terms of flavor changing neutrino interactions

Flavour changing (FC) neutrino-matter interactions have been proposed as a solution to the atmospheric neutrino anomaly. Here we perform the analysis of the full set of the recent 52 kTy Super-Kamiokande atmospheric neutrino data, including the zenith angle distribution of the contained events as well as the higher energy upward-going stopping and through-going muon events. Our results show that the FC mechanism can describe the full data sample with a chi^2_{min}=44/(33 d.o.f) which is acceptable at the 90% confidence level. The combined analysis confines the amount of FC to be either close to maximal or to the level of about (10-50)%.

Can Super-Lamiokande atmospheric neutrino data be explained by flavor-changing induced neutrino oscillations?

We reexamine the possibility of explaining the latest Super-Kamiokande atmospheric neutrino data including the upward going muon data by means of flavor-changing induced massless neutrino oscillations. We show that the energy independence of such flavor-changing coversion clearly leads to incompatibility between the upward going passing muons and the other samples, unless the flux of passing muons is larger than expected.

Tau appearance in atmospheric neutrino interactions

If the correct interpretation of the Super-Kamiokande atmospheric neutrino data is ν μ → ν τ oscillation, the contained data sample should already have more than 10 τ appearance events. We study the challenging task of detecting the τ, focussing on the decay chain τ ±→ ρ ±→ π ± π 0 in events with quasi-elastic τ production. The background level, which is currently quite uncertain because of a lack of relevant neutral current data, can be measured by the near detector in the K2K experiment. Our estimates of the background suggest that it may be possible to detect τ appearance in Super-Kamiokande with 5–10 years of running.

Lepton-flavor violation in the left-handed slepton production at future lepton colliders

The Super-Kamiokande atmospheric neutrino data suggest existence of the large lepton-flavor violating (LFV) interaction in the higher energy scale. If the minimal supersymmetric standard model is extended to have right-handed neutrinos, the left-handed sleptons in the second and third generations are expected to have the LFV masses in the minimal supergravity scenario. In this article we study the LFV signals in the left-handed slepton production at $\mu^+\mu^-$ colliders and $\e^+\e^-$ linear colliders (LC's), $\mu^+\mu^-(\e^+\e^-)\to\tau\mu +4jets + \E$ and $\mu^+\mu^-(\e^+\e^-)\to\tau\mu l+ 2jets+ \E$. The main background comes from decay of a tau lepton into a muon in the lepton-flavor conserving slepton pair production. They are significantly reduced by the energy and the impact parameter cuts for the muon. At $\mu^+\mu^-$ colliders (LC's) it may be possible to reach the mixing angle $\sin 2\theta_{\tilde{\nu}} \gsim 0.1(0.5)$ and the mass difference $\Delta m_{\tilde{\nu}}\gsim 0.1(0.4)$ GeV for the sneutrinos in the second and third generations at the statistical significance of $3\sigma$. Such small mass difference and a mixing angle may be induced radiatively even if the Yukawa coupling constant for the tau neutrino is of the order of 0.1.

L/ E-flatness of the electron-like event ratio, in Super-Kamiokande and a degeneracy, in neutrino masses

We show that the L/ E-flatness of the electron-like event ratio in the Super-Kamiokande atmospheric neutrino data implies the equality of the expectation values for the muon and tau neutrino masses. We establish this result by obtaining a set of three constraints on the neutrino-oscillation mixing matrix as contained in the indicated flatness. The resulting 3×3 neutrino-oscillation matrix depends only on one angle. A remarkable result that follows directly from this matrix is the consistency between the mixing angles observed by LSND and Super-Kamiokande.

Neutrino textures in light of Super-Kamiokande data and a realistic string model

Motivated by the Super-Kamiokande atmospheric neutrino data, we discuss possible textures for Majorana and Dirac neutrino masses within the see-saw framework. There are two main purposes of this paper: first, to gain intuition into this area from a purely phenomenological analysis, and second, to explore to what extent it may be realized in a specific model. We comment initially on the simplified two-generation case, emphasizing that large mixing is not incompatible with a large hierarchy of mass eigenvalues. We also emphasize that renormalization-group effects may amplify neutrino mixing, and we present semi-analytic expressions for estimating this amplification. Several examples are then given of three-family neutrino mass textures, which may also accommodate the persistent solar neutrino deficit, with different assumptions for the neutrino Dirac mass matrices. We comment on a few features of neutrino mass textures arising in models with a U(1) flavour symmetry. Finally, we discuss the possible pattern of neutrino masses in a “realistic” flipped SU(5) model derived from string theory, illustrating how a desirable pattern of mixing may emerge. Both small- or large-angle MSW solutions are possible, while a hierarchy of neutrino masses appears more natural than near-degeneracy. This model contains some unanticipated features that may be relevant in other models also: The neutrino Dirac matrices may not be related closely to the quark mass matrices, and the heavy Majorana states may include extra gauge-singlet fields.

CAN THE SUPER-KAMIOKANDE ATMOSPHERIC DATA PREDICT THE SOLAR NEUTRINO DEFICIT?

In this letter we show that the evidence for neutrino oscillations from the super-Kamiokande atmospheric neutrino data fully determines the 3×3 neutrino-oscillations mixing matrix and predicts an energy-independent solar neutrino deficit at the level of 45%. This corresponds to a ratio of measured to predicted neutrino flux of [Formula: see text], in good agreement with the experimental results. We achieve this result within the framework of a minimal, three-generation neutrino mixing, with mass squared differences of ΔM2≃0.45 eV 2 and [Formula: see text]. The mixing matrix derived here is characterized by the mixing angles θ=35.1°, β=5.5° and ψ=23.3°, and a vanishing CP-violating phase, δ=0.

Big bang nucleosynthesis and active-sterile neutrino mixing: Evidence for maximalνμ↔ντmixing in Super Kamiokande?

Author(s): Shi, X; Fuller, GM | Abstract: We discuss big bang nucleosynthesis constraints on maximal νμ↔νs mixing. Vacuum νμ↔νs oscillation has been proposed as one possible explanation of the Super Kamiokande atmospheric neutrino data. Based on the most recent primordial abundance measurements, we find that the effective number of neutrino species for big bang nucleosynthesis (BBN) is Nv≲3.3. Assuming that all three active neutrinos are light (with masses ≪; 1 MeV), we examine BBN constraints on νμ↔νs mixing in two scenarios: (1) a negligible lepton asymmetry (the standard picture) and (2) the presence of a large lepton asymmetry which has resulted from an amplification by ντ↔νs′, mixing (νs′ being νs or another sterile neutrino species). The latter scenario has been proposed recently to reconcile the BBN constraints and large-angle νμ↔νs mixing. We find that the large-angle νμ↔νs mixing in the first scenario, which would yield Nv≈4, is ruled out as an explanation of the Super Kamiokande data. It is conceivably possible for the νμ↔νs solution to evade BBN bounds in the second scenario, but only if 200eV2≲mντ2-m νs′2≲104 eV2is satisfied, and if ντ decays non-radiatively with a lifetime ≲103 yr. This mass-squared difference implies 15eV≲mντ ≲100 eV if νs is much lighter than ντ. We conclude that maximal (or near maximal) νμ↔ντ mixing is a more likely explanation of the Super Kamiokande data. ©1999 The American Physical Society.