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We present a general framework for models in which the lepton mixing matrix is the product of the maximal mixing matrix U_omega times a matrix constrained by a well-defined Z_2 symmetry. Our framework relies on neither supersymmetry nor non-renormalizable Lagrangians nor higher dimensions; it relies instead on the double seesaw mechanism and on the soft breaking of symmetries. The framework may be used to construct models for virtually all the lepton mixing matrices of the type mentioned above which have been proposed in the literature.
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From the standard seesaw mechanism of neutrino mass generation, which is based on the assumption that the lepton number is violated at a large (~10exp(+15) GeV) scale, follows that the neutrinoless double-beta decay is ruled by the Majorana neutrino mass mechanism. Within this notion, for the inverted neutrino-mass hierarchy we derive allowed ranges of half-lives of the neutrinoless double-beta decay for nuclei of experimental interest with different sets of nuclear matrix elements. The present-day results of the calculation of the neutrinoless double-beta decay nuclear matrix elements are briefly discussed. We argue that if neutrinoless double-beta decay will be observed in future experiments sensitive to the effective Majorana mass in the inverted mass hierarchy region, a comparison of the derived ranges with measured half-lives will allow us to probe the standard seesaw mechanism assuming that future cosmological data will establish the sum of neutrino masses to be about 0.2 eV.
In this paper, we investigate the double covering of modular $Gamma^{}_5 simeq A^{}_5$ group and derive all the modular forms of weight one for the first time. The modular forms of higher weights are also explicitly given by decomposing the direct products of weight-one forms. For the double covering group $Gamma^prime_5 simeq A^prime_5$, there exist two inequivalent two-dimensional irreducible representations, into which we can assign two right-handed neutrino singlets in the minimal seesaw model. Two concrete models with such a salient feature have been constructed to successfully explain lepton mass spectra and flavor mixing pattern. The allowed parameter space for these two minimal scenarios has been numerically explored, and analytically studied with some reasonable assumptions.
We consider the TeV scale left-right symmetric theory which can accommodate low scale seesaw mechanisms consistent with neutrino oscillation data and find new physics contributions to neutrinoless double beta decay. The model facilitates natural type-II seesaw dominance and the presence of extra particles make the Dirac neutrino mass matrix $M_D$ large that leads to large light heavy neutrino mixing. The spontaneous symmetry breaking through doublets, triplets and bidoublet scalars at TeV scale offers rich phenomenology accessible to LHC. From the numerical studies of the new physics contributions to neutrinoless double beta decay we derive a lower limit on absolute scale of lightest neutrino mass and find that normal hierarchy (NH) pattern is favorable taking into account the cosmology and oscillation data.
The see-saw mechanism to generate small neutrino masses is reviewed. After summarizing our current knowledge about the low energy neutrino mass matrix we consider reconstructing the see-saw mechanism. Low energy neutrino physics is not sufficient to reconstruct see-saw, a feature which we refer to as ``see-saw degeneracy. Indirect tests of see-saw are leptogenesis and lepton flavor violation in supersymmetric scenarios, which together with neutrino mass and mixing define the framework of see-saw phenomenology. Several examples are given, both phenomenological and GUT-related. Variants of the see-saw mechanism like the type II or triplet see-saw are also discussed. In particular, we compare many general aspects regarding the dependence of LFV on low energy neutrino parameters in the extreme cases of a dominating conventional see-saw term or a dominating triplet term. For instance, the absence of mu -> e gamma or tau -> e gamma in the pure triplet case means that CP is conserved in neutrino oscillations. Scanning models, we also find that among the decays mu -> e gamma, tau -> e gamma and tau -> mu gamma the latter one has the largest branching ratio in (i) SO(10) type I see-saw models and in (ii) scenarios in which the triplet term dominates in the neutrino mass matrix.
We construct a neutrino mass model based on the flavour symmetry group $A_4times C_4 times C_6 times C_2$ which accommodates a light sterile neutrino in the minimal extended seesaw (MES) scheme. Besides the flavour symmetry, we introduce a $U(1)$ gauge symmetry in the sterile sector and also impose CP symmetry. The vacuum alignments of the scalar fields in the model spontaneously break these symmetries and lead to the construction of the fermion mass matrices. With the help of the MES formulas, we extract the light neutrino masses and the mixing observables. In the active neutrino sector, we obtain the $text{TM}_2$ mixing pattern with non-zero reactor angle and broken $mu$-$tau$ reflection symmetry. We express all the active and the sterile oscillation observables in terms of only four real model parameters. Using this highly constrained scenario we predict $sin^2 theta_{23} =0.545^{+0.003}_{-0.004}$, $sin delta = -0.911^{+0.006}_{-0.005}$, $|U_{e4}|^2 = 0.029^{+0.009}_{-0.008}$, $|U_{mu4}|^2 = 0.010^{+0.003}_{-0.003}$ and $|U_{tau4}|^2 = 0.006^{+0.002}_{-0.002}$ which are consistent with the current data.
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