arXiv subjects

S. Morisi

Publications and source records attributed to S. Morisi.

46 records · Page 3Linked to original sources

arXiv2009

Tri-Bimaximal Lepton Mixing and Leptogenesis

In models with flavour symmetries added to the gauge group of the Standard Model the CP-violating asymmetry necessary for leptogenesis may be related with low-energy parameters. A particular case of interest is when the flavour symmetry produces exact Tri-Bimaximal lepton mixing leading to a vanishing CP-violating asymmetry. In this paper we present a model-independent discussion that confirms this always occurs for unflavoured leptogenesis in type I see-saw scenarios, noting however that Tri-Bimaximal mixing does not imply a vanishing asymmetry in general scenarios where there is interplay between type I and other see-saws. We also consider a specific model where the exact Tri-Bimaximal mixing is lifted by corrections that can be parametrised by a small number of degrees of freedom and analyse in detail the existing link between low and high-energy parameters - focusing on how the deviations from Tri-Bimaximal are connected to the parameters governing leptogenesis.

hep-ph↗

arXiv2009

Phenomenological Consequences of See-Saw in S4 Based Models

It was proposed a flavour model based on the symmetry group S4, managing to describe fermion masses and mixings. The Weinberg operator has been used in order to provide the smallness of the neutrino masses, while a set of scalar fields, getting non-vanishing vacuum expectation values, spontaneously breaks down S4 and provides the Tri-Bimaximal pattern as the lepton mixing matrix. Restricting to this setting, in this paper we analyze possible origins for the effective terms: the type I See-Saw mechanism is the best known approach, but also the type II and III are discussed. The phenomenology related to these models is various and the next future experiments could in principle discriminate among these proposals. Furthermore, we compare our realizations to two relevant A4 based models, also predicting the Tri-Bimaximal lepton mixing, and we find that an analysis on the neutrinoless double beta decay parameters could distinguish among all these realizations. Furthermore a combined measurement of the effective mass and of the lightest neutrino mass could indicate in the next future which is the preferred flavour symmetry group. The introduction of new physics beyond the Standard Model, like heavy right-handed neutrinos, scalar triplets and fermion triplets, let us investigate on leptogenesis and this provides constraints in the realization of the models.

hep-ph↗

arXiv2009

Inverse tri-bimaximal type-III seesaw and lepton flavor violation

We present a type-III version of inverse seesaw or, equivalently an inverse version of type-III seesaw. Naturally small neutrino masses arise at low-scale from the exchange of neutral fermions transforming as hyperchargeless SU(2) triplets. In order to implement tri-bimaximal lepton mixing we supplement the minimal SU(3)xSU(2)xU(1) gauge symmetry with an A4-based flavor symmetry. Our scenario induces lepton flavour violating (LFV) three body decays that can proceed at the tree level, while radiative li to lj gamma decays and mu-e conversion in nuclei are also expected to be sizeable. LFV decays are related by the underlying flavor symmetry and the new fermions are also expected to be accessible for study at the Large Hadron Collider (LHC).

hep-ph↗

arXiv2009

A4-based tri-bimaximal mixing within inverse and linear seesaw schemes

We consider tri-bimaximal lepton mixing within low-scale seesaw schemes where light neutrino masses arise from TeV scale physics, potentially accessible at the Large Hadron Collider (LHC). Two examples are considered, based on the A4 flavor symmetry realized within the inverse or the linear seesaw mechanisms. Both are highly predictive so that in both the light neutrino sector effectively depends only on three mass parameters and one Majorana phase, with no CP violation in neutrino oscillations. We find that the linear seesaw leads to a lower bound for neutrinoless double beta decay while the inverse seesaw does not. The models also lead to potentially sizeable decay rates for lepton flavor violating processes, tightly related by the assumed flavor symmetry.

hep-ph↗

arXiv2009

Tri-Bimaximal lepton mixing with A4 semidirect product Z2 x Z2 x Z2

We consider a model based on A4 semidirect product (Ze x Zmu x Ztau) flavor symmetry for Tri-Bimaximal lepton mixing with two scalar A4-triplets whose vacuum expectation values have the same alignment. Neutrino masses are generated through effective dimension five Weinberg operators. Charged leptons mass hierarchies are generated with an additional Froggatt-Nielsen U(1) flavor symmetry under which right-handed leptons are charged.

hep-ph↗

arXiv2008

Modelling tri-bimaximal neutrino mixing

We model tri-bimaximal lepton mixing from first principles in a way that avoids the problem of the vacuum alignment characteristic of such models. This is achieved by using a softly broken A4 symmetry realized with an isotriplet fermion, also triplet under A4. No scalar A4-triplet is introduced. This represents one possible realization of general schemes characterized by the minimal set of either three or five physical parameters. In the three parameter versions mee vanishes, while in the five parameter schemes the absolute scale of neutrino mass, although not predicted, is related to the two Majorana phases. The model realization we discuss is potentially testable at the LHC through the peculiar leptonic decay patterns of the fermionic and scalar triplets.

hep-ph↗

arXiv2008

Neutrino Physics and A4 matter assignments

Tribimaximal lepton mixing can be obtained assuming A4 flavour symmetry. Many possibilities have been explored in the literature and we give a classification in terms of A4 representations. We propose some phenomenological and theoretical criteria to distinguish between different assignment. As example we consider the possibility to extend A4 to SO(10) grand unified model.

hep-ph↗

arXiv2008

A SUSY A4 model for fermion masses and mixings

We study a supersymmetric extension of the Standard Model based on discrete A4xZ3xZ4 flavor symmetry. We obtain quark mixing angles as well as a realistic fermion mass spectrum and we predict tribimaximal leptonic mixing by a spontaneous breaking of A4. The top quark Yukawa interaction is present at the renormalizable level in the superpotential while all the other Yukawa interactions arise only at higher orders. We study the Higgs potential and show that it can potentially solve the so called vacuum alignment problem. The leading order predictions are not spoiled by subleading corrections.

hep-ph↗

arXiv2008

Tri-bimaximal neutrino mixing and neutrinoless double beta decay

We present a tri-bimaximal lepton mixing scheme where the neutrinoless double beta decay rate (bb0v) has a lower bound which correlates with the ratio alpha = Dmsol/Dmatm well determined by current data, as well as with the unknown Majorana CP phase phi12 characterizing the solar neutrino sub-system. For the special value phi12 = pi/2 (opposite CP-sign neutrinos) the bb0v rate vanishes at tree level when Dmsol/Dmatm = 3/80, only allowed at 3 sigma. For all other cases the rate is nonzero, and lies within current and projected experimental sensitivities close to phi12=0. We suggest two model realizations of this scheme in terms of an A4xZ2 and A4xZ4 flavour symmetries.

hep-ph↗

arXiv2008

Embedding A4 into SU(3)xU(1) flavor symmetry: Large neutrino mixing and fermion mass hierarchy in SO(10) GUT

We present a common explanation of the fermion mass hierarchy and the large lepton mixing angles in the context of a grand unified flavor and gauge theory (GUTF). Our starting point is a SU(3)xU(1) flavor symmetry and a SO(10) GUT, a basic ingredient of our theory which plays a major role is that two different breaking pattern of the flavor symmetry are at work. On one side, the dynamical breaking of SU(3)xU(1) flavor symmetry into U(2)xZ_3 explains why one family is much heavier than the others. On the other side, an explicit symmetry breaking of SU(3) into a discrete flavor symmetry leads to the observed tribimaximal mixing for the leptons. We write an explicit model where this discrete symmetry group is A4. Naturalness of the charged fermion mass hierarchy appears as a consequence of the continuous SU(3) flavor symmetry. Moreover, the same discrete A4-GUT invariant operators are the root of the large lepton mixing, small Cabibbo angle, and neutrino masses.

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