arXiv subjects

Michele Frigerio

Publications and source records attributed to Michele Frigerio.

44 records · Page 3Linked to original sources

arXiv2004

Quaternion Family Symmetry of Quarks and Leptons

To a first approximation, the quark mixing matrix has $θ^q_{13} = θ^q_{23} = 0$, whereas the lepton mixing matrix has $θ^l_{23} = π/4$. We show how this structure may be understood if the family symmetry is $Q_8$, the quaternion group of eight elements. We find three viable scenarios for the Majorana neutrino mass matrix, each depending on 4 parameters and predicting a specific mass spectrum. The phenomenology of the two Higgs doublets which generate the Yukawa sector is analyzed and testable predictions are derived. We discuss also the closely related model based on $D_4$, the symmetry group of the square.

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arXiv2004

Large neutrino mixing and normal mass hierarchy: a discrete understanding

We discuss the possibility of flavor symmetries to explain the pattern of charged lepton and neutrino masses and mixing angles. We emphasize what are the obstacles for the generation of an almost maximal atmospheric mixing and what are the minimal ingredients to obtain it. A model based on the discrete symmetry $S_3$ is constructed, which leads to the dominant $μτ$-block in the neutrino mass matrix, thus predicting normal hierarchy. This symmetry makes it possible to reproduce current data and predicts $0.01\lesssimθ_{13}\lesssim 0.03$ and strongly suppressed neutrinoless $2β$-decay. Moreover, it implies a relation between lepton and quark mixing angles: $θ_{23}^q \approx 2(π/4-θ_{23})$. The Cabibbo mixing can also be reproduced and $θ_{13}^q\sim θ_{12}^qθ_{23}^q$. $S_3$ is thus a candidate to describe all the basic features of Standard Model fermion masses and mixing.

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arXiv2004

Fermion masses in SUSY SO(10) with type II seesaw: a non-minimal predictive scenario

A predictive framework for fermion masses and mixing is given by the supersymmetric SO(10) model with one 10, one bar126, one 126 and one 210 Higgs representations, and type II seesaw dominating the neutrino mass matrix. We investigate the origin of the tension between this model and lepton mixing data and refine previous numerical analyses. We discuss an extension of the minimal model that includes one 120 Higgs chiral superfield representation. This exhausts the possible renormalizable contributions to the Yukawa sector. In spite of the increase in the number of parameters the predictivity of the minimal setting is not spoiled. We argue that the contributions to fermion masses due to the doublet components of 120 can be naturally small compared to those of 10 and 126, thus acting as a perturbation in the fermion mass generation. The antisymmetric nature of the 120 Yukawa coupling affects at leading order the determination of the mixing angles and it allows to remove the inconsistencies between predictions and data on the neutrino parameters. An improvement in the experimental bound on |Ue3| can tell this scenario from the minimal model.

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arXiv2004

Oscillations and leptogenesis: what can we learn about right-handed neutrinos?

Inverting the type-I seesaw formula, we reconstruct the mass matrix of the heavy right-handed neutrinos $N_i$. We analyze how the data on neutrino oscillations affect the structure of this matrix. Under the assumption of hierarchical Dirac-type neutrino masses $m_{Di}$, we compute the mixing angles among $N_i$, their masses $M_i$ and the lepton asymmetries $ε_i$ generated in their decays. Unless special cancellations take place, one finds $M_i\propto m_{Di}^2$ and the generated baryon-to-photon ratio $η_B$ is much smaller than the observed value. We show that successful baryogenesis via leptogenesis occurs in a unique special case, which corresponds to mass degeneracy and maximal mixing of $N_1$ and $N_2$.

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arXiv2003

Probing the seesaw mechanism with neutrino data and leptogenesis

In the framework of the seesaw mechanism with three heavy right-handed Majorana neutrinos and no Higgs triplets we carry out a systematic study of the structure of the right-handed neutrino sector. Using the current low-energy neutrino data as an input and assuming hierarchical Dirac-type neutrino masses $m_{Di}$, we calculate the masses $M_i$ and the mixing of the heavy neutrinos. We confront the inferred properties of these neutrinos with the constraints coming from the requirement of a successful baryogenesis via leptogenesis. In the generic case the masses of the right-handed neutrinos are highly hierarchical: $M_i \propto m_{Di}^2$; the lightest mass is $M_1 \approx 10^3 - 10^6$ GeV and the generated baryon-to-photon ratio $η_B\lesssim 10^{-14}$ is much smaller than the observed value. We find the special cases which correspond to the level crossing points, with maximal mixing between two quasi-degenerate right-handed neutrinos. Two level crossing conditions are obtained: ${m}_{ee}\approx 0$ (1-2 crossing) and $d_{12}\approx 0$ (2-3 crossing), where ${m}_{ee}$ and $d_{12}$ are respectively the 11-entry and the 12-subdeterminant of the light neutrino mass matrix in the basis where the neutrino Yukawa couplings are diagonal. We show that sufficient lepton asymmetry can be produced only in the 1-2 crossing where $M_1 \approx M_2 \approx 10^{8}$ GeV, $M_3 \approx 10^{14}$ GeV and $(M_2 - M_1)/ M_2 \lesssim 10^{-5}$.

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arXiv2003

Radiative corrections to neutrino mass matrix in the Standard Model and beyond

We study the effect of radiative corrections on the structure of neutrino mass matrix. We analyze the renormalization of the matrix from the electroweak scale $m_Z$ to the scale $m_0$ at which the effective operator that gives masses to neutrinos is generated. Apart from Standard Model and MSSM, non-standard extensions of SM are considered at a scale $m_X$ intermediate between $m_Z$ and $m_0$. We find that the dominant structure of the neutrino mass matrix does not change. SM and MSSM corrections produce small (few percents) independent renormalization of each matrix element. Non-standard (flavor changing) corrections can modify strongly small (sub-dominant) matrix elements, which are important for the low energy phenomenology. In particular, we show that all sub-dominant elements can have purely radiative origin, being zero at $m_0$. The set of non-zero elements at $m_0$ can be formed by (i) diagonal elements (unit matrix); (ii) $M_{ee}$ and $M_{μτ}$; (iii)$M_{ee}$ and $μτ$-block elements; (iv) $μτ$-block elements. In the case of unit matrix, both atmospheric and solar mixing angles and mass squared differences are generated radiatively.

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arXiv2002

Structure of neutrino mass matrix and CP violation

We reconstruct the neutrino mass matrix in the flavor basis, using all available experimental data on neutrino oscillations. Majorana nature of neutrinos, normal mass hierarchy (ordering) and validity of the LMA MSW solution of the solar neutrino problem are assumed. We study dependences of the mass matrix elements, m_{alpha beta}, on the CP violating Dirac, delta, and Majorana, rho, sigma, phases, for different values of the mixing angle theta_{13} and of the absolute mass scale, m_1. The contours of constant mass in the rho-sigma plane have been constructed for all m_{alpha beta}. These rho-sigma plots allow to systematically scan all possible structures of the mass matrix. We identify regions of parameters in which the matrix has (i) a structure with the dominant mu tau-block, (ii) various hierarchical structures, (iii) flavor alignment, (iv) structures with special ordering or equalities of elements, (v) the democratic form. In certain cases the matrix can be parameterized by powers of a unique expansion (ordering) parameter lambda approximately 0.2 - 0.3 (lambda_{ord} approximately 0.6 - 0.7). Perspectives to further restrict the structure of mass matrix in future experiments, in particular in the beta beta_{0 nu}-decay searches, are discussed.

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arXiv2002

Structure of neutrino mass matrix

We reconstruct the neutrino mass matrix in the flavor basis, using experimental data on neutrino oscillations. Both normal and inverted hierarchy (ordering) of the mass spectrum are considered. The dependence of the matrix structure on CP violating Majorana phases $ρ$ and $σ$ is studied. The results are shown in $ρ-σ$ plots. We find the possible matrix structures in the different regions of parameters and discuss the perspectives to further restrict the matrix structure in future experiments.

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