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Register a new userRadiative Scaling Neutrino Mass with $A_4$ Symmetry
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A new idea for neutrino mass was proposed recently, where its smallness is not due to the seesaw mechanism, i.e. not inversely proportional to some large mass scale. It comes from a one-loop mechanism with dark matter in the loop consisting of singlet Majorana fermions $N_i$ with masses of order 10 keV and neutrino masses are scaled down from them by factors of about $10^{-5}$. We discuss how this model may be implemented with the non-Abelian discrete symmetry $A_4$ for neutrino mixing, and consider the phenomenology of $N_i$ as well as the extra scalar doublet $(eta^+,eta^0)$.
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A new and radical scenario of the simple 2006 model of radiative neutrino mass is proposed, where there is no seesaw mechanism, i.e. neutrino masses are not inversely proportional to some large mass scale, contrary to the prevalent theoretical thinking. The neutral singlet fermions in the loop have masses of order 10 keV, the lightest of which is absolutely stable and the others are very long-lived. All are components of warm dark matter, which is a possible new paradigm for explaining the structure of the Universe at all scales.
The present work is inspired to execute the $A_4$ modular symmetry in linear seesaw framework by limiting the use of multiple flavon fields. Linear seesaw is acknowledged by extending the Standard Model particle spectrum with six heavy fermions and a singlet scalar. The non-trivial transformation of Yukawa coupling under the $A_4$ modular symmetry helps to explore the neutrino phenomenology with a specific flavor structure of the mass matrix. We discuss the neutrino mixing and obtain the reactor mixing angle and CP violating phase compatible with the observed $3sigma$ region of current oscillation data. Apart, we also collectively investigate the nonzero CP asymmetry from the decay of lightest heavy fermions to explain the preferred phenomena of baryogenesis through leptogenesis
In this paper, we consider a neutrino mass model based on $A_4$ symmetry. The spontaneous symmetry breaking in this model is chosen to obtain tribimaximal mixing in the neutrino sector. We introduce $Z_2 times Z_2$ invariant perturbations in this model which can give rise to acceptable values of $theta_{13}$ and $delta_{CP}$. Perturbation in the charged lepton sector alone can lead to viable values of $theta_{13}$, but cannot generate $delta_{CP}$. Perturbation in the neutrino sector alone can lead to acceptable $theta_{13}$ and maximal CP violation. By adjusting the magnitudes of perturbations in both sectors, it is possible to obtain any value of $delta_{CP}$.
In the context of A_4 symmetry, neutrino tribimaximal mixing is achieved through the breaking of A_4 to Z_3 (Z_2) in the charged-lepton (neutrino) sector respectively. The implied vacuum misalignment of the (1,1,1) and (1,0,0) directions in A_4 space is a difficult technical problem, and cannot be treated without many auxiliary fields and symmetries (and perhaps extra dimensions). It is pointed out here that an alternative scenario exists with A_4 alone and no redundant fields, if neutrino masses are scotogenic, i.e. radiatively induced by dark scalar doublets as recently proposed.
I propose a model of radiative charged-lepton and neutrino masses with $A_4$ symmetry. The soft breaking of $A_4$ to $Z_3$ lepton triality is accomplished by dimension-three terms. The breaking of $Z_3$ by dimension-two terms allow cobimaximal neutrino mixing $(theta_{13} eq 0, theta_{23} = pi/4, delta_{CP} = pm pi/2)$ to be realized with only very small finite calculable deviations from the residual lepton triality. This construction solves a long-standing technical problem inherent in renormalizable $A_4$ models since their inception.
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