OSCILLATIONS OF ATMOSPHERIC NEUTRINOS

Abstract

The observation of muon flavor disappearance in the atmospheric neutrino flux provides compelling evidence for neutrino flavor oscillations and, by implication, for nonzero neutrino rest mass. We review recent results from Super-Kamiokande and other underground atmospheric neutrino experiments. These results, along with oscillation limits from reactor experiments, are evaluated in the context of neutrino three-state mixing. A unifying picture with ν µ to ν τ oscillation as the dominant mode emerges; however, certain ambiguities remain. Future directions for further experimentation are discussed.

Wolfenstein-like parametrization of the neutrino mixing matrix

“…Meanwhile, the K2K long-baseline neutrino experiment [3] has unambiguously observed a reduction of ν µ flux and a distortion of the energy spectrum. These new measurements, together with the robust SNO evidence [4] for the flavor conversion of solar ν e neutrinos and the compelling Super-Kamiokande evidecne [5] for the deficit of atmospheric ν µ neutrinos, convinces us that the hypothesis of neutrino oscillations is indeed correct! We are then led to the exciting conclusion that neutrinos do have masses and lepton flavor mixing does exist.…”

Neutrino mixing and leptogenesis in type-II seesaw scenarios with left–right symmetry

“…It is easy to see that b ′ = b = c ′ = c leads to θ = θ ′ = 0, and thus ε (1) 1 vanishes. Non-zero η B is attributed to non-zero ε (2) 1 in this scenario. Our numerical results are consistent with both the observational data on η B [10] and the experimental data on two neutrino mass-squared differences and three mixing angles [15].…”

“…As one can see from Eq. (10), the CP-violating asymmetry ε 1 depends on the (1, 1), (1,2) and (1, 3) entries ofM † DMD as well as the (1,1) …”

“…Note that ε (1) 1 is the conventional CP-violating term, while ε (2) 1 is due to the one-loop contribution induced by the Higgs triplets [20]. The CP-violating asymmetry ε 1 can give rise to a net lepton number asymmetry in the Universe, and the nonperturbative sphaleron interaction may partially convert this lepton number asymmetry into a net baryon number asymmetry [21],…”

“…Current solar [1], atmospheric [2], reactor [3] and accelerator [4] neutrino oscillation experiments have provided us with very convincing evidence that neutrinos have non-vanishing rest masses and their mixing involves two large angles (θ 12 ∼ 34 • and θ 23 ∼ 45 • ) and one small angle (θ 13 < 10 • ). These important results indicate that the standard electroweak model, in which the gauge group is SU(2) L × U(1) Y and three neutrinos are massless Weyl particles, is actually incomplete.…”