Stefano Morisi

Neutrino mass and mixing: from theory to experiment

The origin of fermion mass hierarchies and mixings is one of the unresolved and most difficult problems in high-energy physics. One possibility to address the flavour problems is by extending the standard model to include a family symmetry. In the recent years it has become very popular to use non-Abelian discrete flavour symmetries because of their power in the prediction of the large leptonic mixing angles relevant for neutrino oscillation experiments. Here we give an introduction to the flavour problem and to discrete groups that have been used to attempt a solution for it. We review the current status of models in light of the recent measurement of the reactor angle, and we consider different model-building directions taken. The use of the flavons or multi-Higgs scalars in model building is discussed as well as the direct versus indirect approaches. We also focus on the possibility of experimentally distinguishing flavour symmetry models by means of mixing sum rules and mass sum rules. In fact, we illustrate in this review the complete path from mathematics, via model building, to experiments, so that any reader interested in starting work in the field could use this text as a starting point in order to obtain a broad overview of the different subject areas.

Model for fermion masses and lepton mixing in SO(10) x A(4)

8 pages.-- PACS nrs.: 11.30.Hv; 12.10.-g; 12.15.Ff; 14.60.Pq.-- ISI Article Identifier: 000246076700079.-- ArXiv pre-print available at: http://arxiv.org/abs/hep-ph/0702034

Fermion masses and mixings in a S4 based model

It has been recently claimed that the symmetry group S4 yields to the Tri-bimaximal neutrino mixing in a “natural” way from the group theory point of view. Approving of this feature as an indication, we build a supersymmetric model of lepton and quark masses based on this family symmetry group. In the lepton sector, a correct mass hierarchy among the charged leptons is achieved together to a neutrino mass matrix which can be diagonalized by the Tri-bimaximal pattern. Our model results to be phenomenologically inequivalent with respect to other proposals based on different flavour groups but still predicting the Tri-bimaximal mixing. In the quark sector a realistic pattern for masses and mixing angles is obtained. The flavour structures of the mass matrices in both the sectors come from the spontaneously symmetry breaking of S4, due to several scalar fields, which get non-zero vacuum expectation values. A specific vacuum alignment is required and it is shown to be a natural results of the minimization of the scalar potential and, moreover, to be stable under the corrections from the higher order terms.

S-4 as a natural flavor symmetry for lepton mixing

Group theoretical arguments seem to indicate the discrete symmetry S{sub 4} as the minimal flavor symmetry compatible with tribimaximal neutrino mixing. We prove in a model-independent way that indeed S{sub 4} can realize exact tribimaximal mixing through different symmetry breaking patterns. We present two models in which lepton tribimaximal mixing is realized in different ways and for each one we discuss the superpotential that leads to the correct breaking of the flavor symmetry.

A SUSY A(4) model for fermion masses and mixings

We study a supersymmetric extension of the Standard Model based on discrete A4 × Z3 × Z4 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.

Phenomenological consequences of the seesaw mechanism in S-4 based models

We thank M. Hirsch and Y. Lin for useful comments and discussions. The work of F. B. has been partially supported by MEC-Valencia MEC Grant No. FPA2008-00319/FPA, by European Commission Contract Nos. MRTN-CT-2004- 503369 and ILIAS/N6 RII3-CT-2004-506222 and by the foundation for Fundamental Research of Matter and the National Organization for Scientific Research. L. M. recognizes that this work has been partly supported by the European Commission under Contract No. MRTN-CT- 2006-035505. The work of S. M. supported by MECValencia MEC Grant No. FPA2008-00319/FPAand by European Commission Contract Nos. MRTN-CT-2004- 503369 and ILIAS/N6 RII3-CT-2004-506222.

Embedding A4 into left–right flavor symmetry: Tribimaximal neutrino mixing and fermion hierarchy

Abstract We address two fundamental aspects of flavor physics: the mass hierarchy and the large lepton mixing angles. On one side, left–right flavor symmetry realizes the democratic mass matrix patterns and explains why one family is much heavier than the others. On the other side, discrete flavor symmetry such as A 4 leads to the observed tribimaximal mixing for the leptons. We show that, by explicitly breaking the left–right flavor symmetry into the diagonal A 4 , it is possible to explain both the observed charged fermion mass hierarchies and quark and lepton mixing angles. In particular we predict a heavy 3rd family, the tribimaximal mixing for the leptons, and we suggest a possible origin of the Cabibbo and other mixing angles for the quarks.

Tribimaximal neutrino mixing and neutrinoless double beta decay

We present a tri-bimaximal lepton mixing scheme where the neutrinoless double beta decay rate has a lower bound which correlates with the ratio α ≡ ∆m2sol/∆m 2 atm well determined by current data, as well as with the unknown Majorana CP phase φ12 characterizing the solar neutrino subsystem. For the special value φ12 = π 2 (opposite CP-sign neutrinos) the ββ0ν rate vanishes at tree level when ∆m2sol/∆m 2 atm = 3/80, only allowed at 3σ. For all other cases the rate is nonzero, and lies within current and projected experimental sensitivities close to φ12 = 0. We suggest two model realizations of this scheme in terms of an A4 × Z2 and A4 × Z4 flavour symmetries.

Model for T2K indication with maximal atmospheric angle and tri-maximal solar angle

Recently T2K experiment gives hint in favor of large reactor angle

Discrete dark matter

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