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Register a new userSequentially loop supressed fermion masses from a unique discrete symmetry
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Publication date
2019
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English
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We propose a systematic and renormalizable sequential loop suppression mechanism to generate the hierarchy of the Standard Model fermion masses from one discrete symmetry. The discrete symmetry is sequentially softly broken in order to generate one-loop level masses for the bottom, charm, tau and muon leptons and two-loop level masses for the lightest Standard Model charged fermions. The tiny masses for the light active neutrinos are produced from radiative type-I seesaw mechanism, where the Dirac mass terms are effectively generated at two-loop level.
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We have built a renormalizable $U(1)_X$ model with a $Sigma (18)times Z_4$ symmetry, whose spontaneous breaking yields the observed SM fermion masses and fermionic mixing parameters. The tiny masses of the light active neutrinos are produced by the type I seesaw mechanism mediated by very heavy right handed Majorana neutrinos. To the best of our knowledge, this model is the first implementation of the $Sigma (18)$ flavor symmetry in a renormalizable $U(1)_X$ model. Our model allows a successful fit for the SM fermion masses, fermionic mixing angles and CP phases for both quark and lepton sectors. The obtained values for the physical observables of both quark and lepton sectors are in accordance with the experimental data. We obtain an effective neutrino mass parameter of $langle m_{ee}rangle=1.51times 10^{-3}, mathrm{eV}$ for normal ordering and $langle m_{ee}rangle =4.88times 10^{-2} , mathrm{eV}$ for inverted ordering which are well consistent with the recent experimental limits on neutrinoless double beta decay.
In a recently proposed multi-Higgs extension of the standard model in which discrete symmetries, $A_4$ and $Z_3$ are imposed we show that, after accommodating the fermion masses and the mixing matrices in the charged currents, the mixing matrices in the neutral currents induced by neutral scalars are numerically obtained. However, the flavor changing neutral currents are under control mainly by mixing and/or mass suppressions in the neutral scalar sector.
Minimal Flavour Violation (MFV) postulates that the only source of flavour changing neutral currents and CP violation, as in the Standard Model, is the CKM matrix. However it does not address the origin of fermion masses and mixing and models that do usually have a structure that goes well beyond the MFV framework. In this paper we compare the MFV predictions with those obtained in models based on spontaneously broken (horizontal) family symmetries, both Abelian and non-Abelian. The generic suppression of flavour changing processes in these models turns out to be weaker than in the MFV hypothesis. Despite this, in the supersymmetric case, the suppression may still be consistent with a solution to the hierarchy problem, with masses of superpartners below 1 TeV. A comparison of FCNC and CP violation in processes involving a variety of different family quantum numbers should be able to distinguish between various family symmetry models and models satisfying the MFV hypothesis.
In an unconventional realization of left-right symmetry, the particle corresponding to the left-handed neutrino nu_L (with SU(2)_L interactions) in the right-handed sector, call it n_R (with SU(2)_R interactions), is not its Dirac mass partner, but a different particle which may be a dark-matter candidate. In parallel to leptogenesis in the SU(2)_L sector, asymmetric production of n_R may occur in the SU(2)_R sector. This mechanism is especially suited for n_R mass of order 1 to 10 keV, i.e. warm dark matter, which is a possible new paradigm for explaining the structure of the Universe at all scales.
A short review of the status of supersymmetric grand unified theories and their relation to the issue of fermion masses and mixings is given.
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