We study the role that the future detection of the neutrino burst from a galactic supernova can play in the reconstruction of the neutrino mass spectrum. We consider all possible 3ν mass and flavor spectra which describe the solar and atmospheric neutrino data. For each of these spectra we find the observable effects of the supernova neutrino conversions both in the matter of the star and the earth. We show that studies of the electron neutrino and anineutrino spectra as well as observations of the neutral current effects from supernova will allow us (i) to identify the solar neutrino solution, (ii) to determine the type of mass hierarchy (normal or inverted) and (iii) to probe the mixing |U e3 | 2 to values as low as 10 −4 − 10 −3 .

We propose a new mechanism of leptogenesis in which the asymmetries in lepton numbers are produced through the CP-violating oscillations of "sterile" (electroweak singlet) neutrinos. The asymmetry is communicated from singlet neutrinos to ordinary leptons through their Yukawa couplings. The lepton asymmetry is then reprocessed into baryon asymmetry by electroweak sphalerons. We show that the observed value of baryon asymmetry can be generated in this way, and the masses of ordinary neutrinos induced by the seesaw mechanism are in the astrophysically and cosmologically interesting range. Except for singlet neutrinos, no physics beyond the Standard Model is required. 1. The origin of the excess of baryons over anti-baryons in the Universe remains one of the fascinating problems of particle physics and cosmology. A number of mechanisms have been proposed to date to explain this asymmetry (for recent reviews see, e.g., [1]). One of the simplest possibilities, suggested by Fukugita and Yanagida [2], is that the baryon asymmetry has originated from physics in the leptonic sector. Namely, it was assumed that at temperatures well above the electroweak scale, lepton asymmetry was produced, which was then reprocessed into the baryon asymmetry by non-perturbative electroweak effects [3] -sphalerons [4]. According to ref.[2] the lepton asymmetry is generated in out-of-equilibrium, CP-and lepton number non-conserving decays of heavy Majorana neutrinos (for recent discussions see, e.g., ref.[5] and references therein).In this Letter we propose a new realization of baryogenesis through leptogenesis which also makes use of the electroweak reprocessing of the lepton number into the baryon number. Like the Fukugita-Yanagida mechanism, our proposal requires only mild extension of the Standard Model by introducing "sterile" (i.e., electroweak singlet) heavy neutrinos. However, our mechanism of leptogenesis is entirely different from that of ref.[2]: we suggest that asymmetries in lepton numbers were generated due to oscillations of these singlet neutrinos and their interactions with ordinary matter in the early Universe. Moreover, the novel feature of our scenario is that the total lepton number is not violated in these oscillations and/or interactions; an important ingredient is separation (rather than generation) of lepton number, i.e., its redistribution between different species of singlet neutrinos.For this reason we do not necessarily require that singlet neutrinos be Majorana particles; Dirac "sterile" neutrinos are equally suitable (and even better in some respect) for our mechanism to work. Furthermore, in our case the values of the masses and couplings of singlet neutrinos are very different from those of ref.[2].2. Let us consider the Standard Model extended by adding three types of Majorana neutrinos N a , a = A, B, C which interact with other particles only through their Yukawa couplings [6]. The corresponding Lagrangian can be written in the "Yukawa basis" (where the matrix of Yukawa coupling constants has been diagon...

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We determine the neutrino parameters for MSW and vacuum oscillations (active and sterile neutrinos) that are allowed by the separate, and collective, imposition of the constraints from total event rates in the chlorine, GALLEX, SAGE, and SuperKamiokande experiments (504 days), the SuperKamiokande electron energy spectrum, and the SuperKamiokande zenith-angle dependence. The small mixing angle MSW solution is acceptable at the 7% C.L. (8% for sterile nu's) and the vacuum solution is acceptable at the 6% C.L. . The best-fit global MSW solution for active neutrinos is: Delta m^2 = 5 x 10^-6 eV^2, sin^2 (2 theta) = 5.5 x 10^{-3} (and for sterile neutrinos: Delta m^2 = 4 x 10^-6 eV^2, sin^2 (2 theta) = 7 x 10^-3). For vacuum oscillations, the best-fit solution is: Delta m^2 = 6.5 x 10^-11 eV^2, sin^2 (2 theta) = 0.75 . An arbitrary combination of undistorted (no oscillations) pp, 7Be, 8B, and CNO neutrino fluxes is inconsistent with the combined data sets at the 3.5 sigma C.L., independent of astrophysical considerations. We use improved calculations of solar model fluxes, neutrino absorption cross sections and energy spectra, and a detailed evaluation of regeneration effects.Comment: LaTeX file. Added Figure comparing with SuperK spectrum. Predictions for LENS experiment. Viewgraphs and related information at http://www.sns.ias.edu/~jn

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We review the present state and future outlook of our understanding of neutrino masses and mixings. We discuss what we think are the most important perspectives on the plausible and natural scenarios for neutrinos and attempt to throw light onto the flavor problem of quarks and leptons. This review focuses on the seesaw mechanism, which fits into a big picture of particle physics such as supersymmetry and grand unification providing a unified approach to the flavor problem of quarks and leptons. We argue that in combination with family symmetries, this may be at the heart of a unified understanding of the flavor puzzle. We also discuss other new physics ideas such as neutrinos in models with extra dimensions and possible theoretical implications of sterile neutrinos. We outline some tests for the various schemes.

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We consider the possibility to detect right-handed neutrinos, which are mostly singlets of the Standard Model gauge group, at future accelerators. Substantial mixing of these neutrinos with the active neutrinos requires a cancellation of different contributions to the light neutrino mass matrix at the level of 10 −8 . We discuss possible symmetries behind this cancellation and argue that for three right-handed neutrinos they always lead to conservation of total lepton number. Light neutrino masses can be generated by small perturbations violating these symmetries. In the most general case, LHC physics and the mechanism of neutrino mass generation are essentially decoupled; with additional assumptions, correlations can appear between collider observables and features of the neutrino mass matrix.