Abstract
We propose a minimal predictive inverse seesaw model based on two right-handed neutrinos and two additional singlets, leading to the same low energy neutrino mass matrix as in the Littlest Seesaw (LS) (type I) model. In order to implement such a Littlest Inverse Seesaw (LIS) model, we have used an S4 family symmetry, together with other various symmetries, flavons and driving fields. The resulting LIS model leads to an excellent fit to the low energy neutrino parameters, including the prediction of a normal neutrino mass ordering, exactly as in the usual LS model. However, unlike the LS model, the LIS model allows charged lepton flavor violating (CLFV) processes and lepton conversion in nuclei within reach of the forthcoming experiments.
Highlights
The existence of three fermion families, as well as their particular pattern of masses and mixing angles is not explained in the Standard Model (SM), and makes it appealing to consider a more fundamental theory addressing these issues
In this paper, motivated by such considerations, we propose a fusion of the Littlest Seesaw (LS) model and the inverse seesaw model [33], which we refer to as the Littlest Inverse Seesaw (LIS) model
In the same region of parameter space, we found that the branching ratios for the τ → μγ and τ → eγ decays are in the ranges 2 × 10−13 Br (τ → μγ) 1.6 × 10−12 and 2 × 10−14 Br (τ → eγ) 1.8 × 10−13, respectively, which is well below their upper experimental limits of 4.4 × 10−9 and 3.3 × 10−9, respectively
Summary
The existence of three fermion families, as well as their particular pattern of masses and mixing angles is not explained in the Standard Model (SM), and makes it appealing to consider a more fundamental theory addressing these issues. The above mass matrix structures are motivated by the phenomenological success of the low energy mass matrix in Eq 1.3 which is identical to that of the usual LS model, involving two right-handed neutrinos, but in this case arising from the inverse seesaw model, including the two additional singlets Such an extension allows CLFV decays, such as μ → eγ, at observable rates, since in the inverse seesaw model small neutrino masses are explained by the smallness of the μ matrix 1, which allows Dirac masses to be large even for TeV scale values of M. The superpotential that determines the vacuum configuration for the S4 doublet and triplet scalars of our model is presented in Appendix B
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