Special Symmetric Quark Mass Matrices

We give a procedure to construct a special class of symmetric quark mass matrices near the democratic limit of equal Yukawa couplings for each sector. It is shown that within appropriate weak-bases, the requirements of symmetry and arg(det[M])=0 are very strong conditions, that necessarily lead to a Cabibbo angle given by |V_us|=Sqrt[md/ms], and to |V_cb|~ms/mb, in first order. In addition, we prove that the recently classified ansatze, which also reproduce these mixing relations, and which were based on the hypothesis of the Universal Strength for Yukawa couplings, where all Yukawa couplings have equal moduli while the flavour dependence is only in their phases, are, in fact, particular cases of the generalized symmetric quark mass matrix ansatze we construct here. In an excellent numerical example, the experimental values on all quark mixings and masses are accommodated, and the CP violation phase parameter is shown to be crucially dependent on the values of mu and V_us.Comment: 13 pages, LaTe

Quasi degenerate Neutrino Masses with Universal Strength Yukawa Couplings

A simple ansatz is proposed for neutrino and charged lepton mass matrices, within the framework of universal strength for Yukawa couplings. In this framework all Yukawa couplings have equal moduli and the flavour dependence is only in their phases. We take into account the solar neutrino deficit and the atmospheric neutrino anomaly, assuming three neutrino families only. The ansatz leads in a natural way to small mixing involving neutrinos of quasi degenerate masses, as required to explain the solar neutrino deficit in the nonadiabatic MSW solution, while having the large mixing necessary to explain the atmospheric neutrino anomaly.Comment: 10 pages, LATEX, no special macros, no figures, no table

Symmetries, Large Leptonic Mixing and a Fourth Generation

We show that large leptonic mixing occurs most naturally in the framework of the Sandard Model just by adding a fourth generation. One can then construct a small $Z_4$ discrete symmetry, instead of the large $S_{4L}\times S_{4R}$, which requires that the neutrino as well as the charged lepton mass matrices be proportional to a $4\times 4$ democratic mass matrix, where all entries are equal to unity. Without considering the see-saw mechanism, or other more elaborate extensions of the SM, and contrary to the case with only 3 generations, large leptonic mixing is obtained when the symmetry is broken.Comment: 6 pages, ReVTeX, no figure

Natural quasi-alignment with two Higgs doublets and RGE stability

In the context of two Higgs doublet models, we study the conditions required in order to have stable quasi-alignment in flavour space. We show that stability under the renormalisation group equations imposes strong constraints on the flavour structure of the Yukawa couplings associated to each one of the Higgs doublets. In particular, we find a novel solution, where all Yukawa couplings are proportional to the so-called democratic matrix. This solution is rather unique, since it is the only stable solution which is a good starting point for reproducing the observed pattern of quark masses and mixing. We also showthat this stable solution can be obtained by imposing on the Lagrangian a Z(3)xZ(3)' flavour symmetry. Quark masses of the lighter quark generations are generated through the breaking of this discrete symmetry, and, at this stage, scalar-mediated flavour-changing neutralcurrents arise, but they are naturally suppressed by the smallness of the light quark masses. In this way, we relate Higgs alignment to the hierarchy of the quark masses through a discrete family symmetry

The Problem of Large Leptonic Mixing

Unlike in the quark sector where simple $S_3$ permutation symmetries can generate the general features of quark masses and mixings, we find it impossible (under conditions of hierarchy for the charged leptons and without considering the see-saw mechanism or a more elaborate extension of the SM) to guarantee large leptonic mixing angles with any general symmetry or transformation of only known particles. If such symmetries exist, they must be realized in more extended scenarios.Comment: RevTeX, 4 pages, no figure

The Symmetry behind Extended Flavour Democracy and Large Leptonic Mixing

We show that there is a minimal discrete symmetry which leads to the extended flavour democracy scenario constraining the Dirac neutrino, the charged lepton and the Majorana neutrino mass term ($M_R$) to be all proportional to the democratic matrix, with all elements equal. In particular, this discreet symmetry forbids other large contributions to $M_R$, such as a term proportional to the unit matrix, which would normally be allowed by a $S_{3L}\times S_{3R}$ permutation symmetry. This feature is crucial in order to obtain large leptonic mixing, without violating 't Hooft's, naturalness principle.Comment: 11 pages, LaTe

CP-odd invariants in models with several Higgs doublets

We present CP-odd Higgs-basis invariants, which can be used to signal CP violation in a multi-Higgs system, written in an arbitrary Higgs basis. It is shown through specific examples how these CP-odd invariants can also be useful to determine the character of CP breaking (i.e. whether it is hard or soft CP breaking) in a given Higgs Lagrangian. We analyse in detail the cases of two and three Higgs doublets

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.Comment: LaTeX, 17 pages, 2 eps figures. A section on RGE evolution and a few references added; minor typos correcte

Invariants, Alignment and the Pattern of Fermion Masses and Mixing

We show that the main features of the pattern of fermion masses and mixing can be expressed in terms of simple relations among weak-basis invariants. In the quark sector, we identify the weak-basis invariants which signal the observed alignment of the up and down quark mass matrices in flavour space. In the lepton sector, we indicate how a set of conditions on weak-basis invariants can lead to an approximate tribimaximal lepton mixing matrix. We also show the usefulness of these invariants in the study of specific ans\"atze for the flavour structure of fermion mass matrices.Comment: 16 pages, 1 figur

Universality of Yukawa Couplings Confronts Recent Neutrino Data

We propose a flavour structure for the leptonic sector of the Standard Model, based on the idea of universality of Yukawa couplings, which accommodates all the experimental data on neutrino masses and mixing, at the same time predicting specific correlations between low energy measurable quantities, such as the ratio of neutrino squared mass differences, $|U_{13}|$, the leptonic Dirac phase, and the double-beta decay mass parameter. We also point out that it is possible, in this framework, to generate a sufficient amount of baryon asymmetry of the Universe through leptogenesis.Comment: 22 Pages, 5 Figure