Narrow Results

We consider an extension of the Standard Model with three right-handed (RH) neutrinos and a Dirac pair of extra sterile neutrinos, odd under a discrete $Z_2$ symmetry, in order to have left–right symmetry in the neutrino content and obtain tiny neutrino masses from the latter ones only. Our working hypothesis is that the heavy RH neutrinos do not influence phenomenology at low energies. We use the usual high-scale seesaw to suppress all of the mass terms involving RH neutrinos and a low-scale minimal variant of the linear seesaw led by the Dirac mass of the extra sterile neutrinos to provide the small mass of active neutrinos. One of the active neutrinos is massless, which fixes the mass of the other two on the basis of a soft breaking of the $Z_2$ symmetry. The mixing between the extra neutrinos makes for a particle that effectively behaves like a Dirac sterile neutrino with mass around the GeV level.

We explore an $A_4$ flavor-based left–right symmetric model where neutrino masses and mixing are explained through linear seesaw mechanism. Taking $A_4$ flavor symmetry and left–right gauge symmetry together and adding a low-scale seesaw mechanism gives the advantage of (a) getting diagonal structures for both charged lepton mass matrix and Dirac neutrino mass matrix, (b) simpler relations among neutrino observables in the model and (c) large light-heavy neutrino mixing which gives dominant contributions to various lepton flavor violating decays like $\mu \to e\gamma$, $\mu \to eee$, $\mu \to e$. By saturating the experimental bounds on these decays we derive constraints on input model parameters and study analytically as well as numerically the correlation among model parameters and their dependence on experimentally determined neutrino parameters like ${𝜃}_{12}$, ${𝜃}_{23}$, ${𝜃}_{13}$, $\mathrm{\Delta }{m}_{\mathrm{\text{atm}}}^2$, $\mathrm{\Delta }{m}_{\mathrm{\text{sol}}}^2$. We also study CP-violation via Jarlskog invariants and show extra contributions to CP-violating effects that the model generates due to unitarity violation in the neutrino sector.