Light Neutrino Mass Matrix Research Articles

Properties of fermion mixings in intersecting D-brane models

We consider the Yukawa couplings for quarks and leptons in the context of Pati–Salam model using intersecting D-brane models where the Yukawa coupling matrices are rank one in a simple choice of family replication. The CKM mixings can be explained by perturbing the rank 1 matrix using higher order terms involving new Higgs fields available in the model. We show that the near bi-large neutrino mixing angles can be naturally explained, choosing the light neutrino mass matrix to be type II seesaw dominant. The predicted value of Ue3 is in the range ≃0.05–0.15. In the quark sector, Vcb is naturally close to the strange/bottom quark mass ratio and we obtain an approximate relation VubVcb≃(ms/mb)2Vus. The geometrical interpretations of the neutrino mixings are also discussed.

Screening of Dirac flavor structure in the seesaw and neutrino mixing

We consider the mechanism of screening of the Dirac flavor structure in the context of the double seesaw mechanism. As a consequence of screening, the structure of the light neutrino mass matrix, m_\nu, is determined essentially by the structure of the (Majorana) mass matrix, M_S, of new super-heavy (Planck scale) neutral fermions S. We calculate effects of the renormalization group running in order to investigate the stability of the screening mechanism with respect to radiative corrections. We find that screening is stable in the supersymmetric case, whereas in the standard model it is unstable for certain structures of M_S. The screening mechanism allows us to reconcile the (approximate) quark-lepton symmetry and the strong difference of the mixing patterns in the quark and lepton sectors. It opens new possibilities to explain a quasi-degenerate neutrino mass spectrum, special ``neutrino'' symmetries and quark-lepton complementarity. Screening can emerge from certain flavor symmetries or Grand Unification.

A flavor symmetry for quasi-degenerate neutrinos: $L_\mu - L_\tau$

We consider the flavor symmetry L_mu - L_tau for the neutrino mass matrix. The most general neutrino mass matrix conserving L_mu - L_tau predicts quasi-degenerate neutrino masses with one maximal and two zero mixing angles. The presence of L_mu - L_tau can also be motivated by the near-bimaximal form of the neutrino mixing matrix. Furthermore, it is a special case of mu-tau symmetric mass matrices. Breaking the flavor symmetry by adding a small flavor-blind term to the neutrino mass matrix and/or by applying radiative corrections is shown to reproduce the observed neutrino oscillation phenomenology. Both the normal and inverted mass ordering can be accommodated within this scheme. Moderate cancellation for neutrinoless double beta decay is expected. The observables U_{e3}^2 and |1/2 - \sin^2\theta_{23}| are proportional to the inverse of the fourth power of the common neutrino mass scale. We comment on whether the atmospheric neutrino mixing is expected to lie above or below pi/4. We finally present a model based on the see-saw mechanism which generates a light neutrino mass matrix with an (approximate) L_mu - L_tau flavor symmetry. This is a minimal model with just one standard Higgs doublet and three heavy right-handed neutrinos. It needs only small values for the soft L_mu - L_tau breaking terms to reproduce the phenomenological viable mass textures analyzed.

A simple model of neutrino masses from supersymmetry breaking

We analyze a class of supersymmetric models first introduced by Arkani-Hamed et al. and Borzumati et al. in which the light neutrino masses result from higher-dimensional supersymmetry-breaking terms in the MSSM super-potentials and Kahler potentials. The mechanism is closely related to the Giudice–Masiero mechanism for the MSSM μ parameter, and leads to TeV-scale right-handed neutrino and sneutrino states, that are in principle accessible to direct experimental study. The dominant contribution to the light neutrino (Majorana) mass matrix is a one-loop term induced by a lepton-number violating B-term for the sneutrino states that is naturally present. We focus upon the simplification and analysis of the flavour structure of this general class of models, finding that simple and novel origins for the light neutrino mass matrix are possible. We find that a subdominant tree-level ‘see-saw’ contribution may lead to interesting perturbations of the leading one-loop-induced flavour structure, possibly generating the small ratio Δmsolar2/Δmatm2 dynamically.

A simple model of neutrino masses from supersymmetry breaking

We analyze a class of supersymmetric models first introduced by Arkani-Hamed et al. and Borzumati et al. in which the light neutrino masses result from higher-dimensional supersymmetry-breaking terms in the MSSM super-potentials and Kahler potentials. The mechanism is closely related to the Giudice–Masiero mechanism for the MSSM μ parameter, and leads to TeV-scale right-handed neutrino and sneutrino states, that are in principle accessible to direct experimental study. The dominant contribution to the light neutrino (Majorana) mass matrix is a one-loop term induced by a lepton-number violating B -term for the sneutrino states that is naturally present. We focus upon the simplification and analysis of the flavour structure of this general class of models, finding that simple and novel origins for the light neutrino mass matrix are possible. We find that a subdominant tree-level ‘see-saw’ contribution may lead to interesting perturbations of the leading one-loop-induced flavour structure, possibly generating the small ratio Δ m solar 2 /Δ m atm 2 dynamically.

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 $\eta_B\lesssim 10^{-14}$ 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 $(M_2 - M_1)/ M_2 \lesssim 10^{-5}$.

One-loop corrections to the seesaw mechanism in the multi-Higgs-doublet standard model

We consider the lepton sector of the standard model and allow for an arbitrary number of Higgs doublets and, moreover, for the presence of right-handed neutrino singlets which enable the seesaw mechanism. In this framework, we identify and calculate the dominant one-loop radiative corrections to the tree-level mass matrix of the light neutrinos. The interesting feature is that both the tree-level and the one-loop contributions to the light-neutrino mass matrix are quadratic in the Yukawa couplings, with the effect that the one-loop contribution is smaller than the tree-level one mainly because of the one-loop factor (16π2)−1. We also point out the possibility of generating radiatively—in this framework—the ratio of solar over atmospheric neutrino mass-squared differences, as needed for the large-mixing-angle MSW solution of the solar-neutrino problem.

Realization of the large mixing angle solar neutrino solution in anSO(10)supersymmetric grand unified model

An SO(10) supersymmetric grand unified model proposed earlier leading to the solar solution involving ``just-so'' vacuum oscillations is reexamined to study its ability to obtain the other possible solar solutions. It is found that all four viable solar neutrino oscillation solutions can be achieved in the model simply by modification of the right-handed Majorana neutrino mass matrix, M_R. Whereas the small mixing and vacuum solutions are easily obtained with several texture zeros in M_R, the currently-favored large mixing angle solution requires a nearly geometric hierarchical form for M_R that leads by the seesaw formula to a light neutrino mass matrix which has two or three texture zeros. The form of the matrix which provides the ``fine-tuning'' necessary to achieve the large mixing angle solution can be understood in terms of Froggatt-Nielsen diagrams for the Dirac and right-handed Majorana neutrino mass matrices. The solution fulfils several leptogenesis requirements which in turn can be responsible for the baryon asymmetry in the universe.

Neutrino masses and mixing with seesaw mechanism and universal breaking of extended democracy

In the framework of a minimal extension of the SM, where the only additional fields are three right-handed neutrinos, we suggest that the charged lepton, the Dirac neutrino and the right-handed Majorana neutrino mass matrices are all, to leading approximation, proportional to the democratic matrix. With the further assumption that the breaking of this extended democracy is universal for all leptonic mass matrices, a large mixing in the 2–3 sector can be obtained and is linked to the seesaw mechanism, together with the existence of a strong hierarchy in the masses of right-handed neutrinos. The structure of the resulting effective mass matrix of light neutrinos is stable against the RGE evolution, and a good fit to all solar and atmospheric neutrino data is obtained.

Predictive minimal model for neutrino masses and mixings

A model is considered in which the scale of the heavy singlet neutrinos is a few orders of magnitude below the grand unification scale and where right-handed vector bosons still play a negligible role. In a basis with diagonal up-quark and Dirac-neutrino mass matrices it is assumed that the heavy neutrino mass matrix has only zero elements in its diagonal, in analogy with the light neutrino mass matrix in the Zee model. Connecting then the remaining matrix elements with the small parameter describing the hierarchy of quark masses and mixings and by assuming commutativity of the charged lepton with the down-quark mass matrix, the calculation of all neutrino properties can be performed in terms of the two mass differences relevant for atmospheric and solar neutrino oscillations. $\mathrm{CP}$ violation is directly related to $\mathrm{CP}$ violation in the quark sector.

Large mixing angle MSW and atmospheric neutrinos from single right-handed neutrino dominance and U(1) family symmetry

Single right-handed neutrino dominance (SRHND) in the 23 sector of the light effective neutrino mass matrix has been proposed as a natural explanation for the concurrent large 23 mixing angle and large 23 mass hierarchy. In this paper we show how large 12 mixing angles, suitable for the large mixing angle (LMA) MSW solution to the solar neutrino problem, may arise from SRHND. In order to understand the conditions for SRHND and LMA MSW we first consider the case of one and two right-handed neutrinos, and obtain simple analytic conditions which are then extended to the case of three right-handed neutrinos. We then introduce a single U(1) family symmetry and show how these analytic conditions may be translated into U(1) charge assignments and perform a systematic search for the simplest examples.

Unified explanation of the solar and atmospheric neutrino puzzles in a supersymmetric SO(10) model

It was recently suggested that in a class of supersymmetric SO(10) models with Higgs multiplets in $10$, and a single $126+\overline{126}$ representation, if the $\overline{126}$ contributes both to the right handed neutrino masses as well as to the charged fermion masses, one can have a complete prediction of the neutrino masses and mixings. It turns out that if one chooses only one $10$, there are no regions in the parameter space where one can have a large ${\ensuremath{\nu}}_{\ensuremath{\mu}}$-${\ensuremath{\nu}}_{\ensuremath{\tau}}$ mixing angle necessary to solve the atmospheric neutrino deficit while at the same time solving the solar neutrino puzzle via the ${\ensuremath{\nu}}_{e}\ensuremath{\leftrightarrow}{\ensuremath{\nu}}_{\ensuremath{\mu}}$ oscillation. We show that this problem can be solved in a particular class of SO(10) models with a pair of $10$ multiplets if we include the additional left-handed triplet contribution to the light neutrino mass matrix. This model cannot reproduce the mass and mixing parameters required to explain the Liquid Scintillation Neutrino Detector observations nor does it have neutrino hot dark matter.