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...

We present a number of complete sets of series expansion formulas for neutrino oscillation probabilities in matter of constant density for three flavors. In particular, we study expansions in the mass hierarchy parameter α ≡ ∆m 2 21 /∆m 2 31 and mixing parameter s 13 ≡ sin θ 13 up to second order and expansions only in α and only in s 13 up to first order. For each type of expansion we also present the corresponding formulas for neutrino oscillations in vacuum. We perform a detailed analysis of the accuracy of the different sets of series expansion formulas and investigate which type of expansion is most accurate in different regions of the parameter space spanned by the neutrino energy E, the baseline length L, and the expansion parameters α and s 13 . We also present the formulas for series expansions in α and in s 13 up to first order for the case of arbitrary matter density profiles. Furthermore, it is shown that in general all the 18 neutrino and antineutrino oscillation probabilities can be expressed through just two independent probabilities. a

arXiv:hep-ph/9507275v1 10 Jul 1995 IC/95/125 TUM-HEP-221/95 MPI-PhT/95-35 FTUV/95-34, IFIC/95-36 AbstractWe study left-right symmetric models which contain only fermion and gauge boson fields and no elementary scalars. The Higgs bosons are generated dynamically through a set of gauge-and parity-invariant 4-fermion operators. It is shown that in a model with a composite bi-doublet and two triplet scalars there is no parity breaking at low energies, whereas in the model with two doublets instead of two triplets parity is broken automatically regardless of the choice of the parameters of the model. For phenomenologically allowed values of the right-handed scale a tumbling symmetry breaking mechanism is realized in which parity breaking at a high scale µ R propagates down and eventually causes the electro-weak symmetry breaking at the scale µ EW ∼ 100 GeV . The model exhibits a number of low and intermediate mass Higgs bosons with certain relations between their masses. In particular, the components of the SU(2) L Higgs doublet χ L are pseudo-Goldstone Bosons of an accidental (approximate) SU(4) symmetry of the Higgs potential and therefore are expected to be relatively light.

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We present an exact analytic solution to the neutrino evolution equation in matter with periodic step-function density profile and discuss in detail the parametric resonance of neutrino oscillations that can occur in such a system. Solar and at- If observed, the enhanced day-night effect for core crossing neutrinos would therefore confirm both the MSW solution of the solar neutrino problem and the parametric resonance of neutrino oscillations.

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