Abstract
We study the phenomenology of the minimal (2, 2) inverse-seesaw model supplemented with Abelian flavour symmetries. To ensure maximal predictability, we establish the most restrictive flavour patterns which can be realised by those symmetries. This setup requires adding an extra scalar doublet and two complex scalar singlets to the Standard Model, paving the way to implement spontaneous CP violation. It is shown that such CP-violating effects can be successfully communicated to the lepton sector through couplings of the scalar singlets to the new sterile fermions. The Majorana and Dirac CP phases turn out to be related, and the active-sterile neutrino mixing is determined by the active neutrino masses, mixing angles and CP phases. We investigate the constraints imposed on the model by the current experimental limits on lepton flavour-violating decays, especially those on the branching ratio BR(μ → eγ) and the capture rate CR(μ − e, Au). The prospects to further test the framework put forward in this work are also discussed in view of the projected sensitivities of future experimental searches sensitive to the presence of heavy sterile neutrinos. Namely, we investigate at which extent upcoming searches for μ → eγ, μ → 3e and μ − e conversion in nuclei will be able to test our model, and how complementary will future high-energy collider and beam-dump experiments be in that task.
Highlights
In this paper we have thoroughly investigated the minimal inverse-seesaw mechanism with couplings constrained by U(1) flavour symmetries, and with all fermion masses generated via spontaneous symmetry breaking through vacuum expectation value (VEV) of doublet and singlet scalar fields
After finding the maximally-restrictive mass matrices compatible with present neutrino data, we have identified all possible U(1) symmetry realisations and concluded that at least two Higgs doublets and two complex scalar singlets are required to successfully implement those symmetries
The presence of such singlets opens up the possibility for spontaneous CPV (SCPV), which turns out to be successfully communicated to the lepton sector via their couplings to the new sterile fermions
Summary
The ISS mechanism can be implemented by extending the SM particle content with nR RH neutrinos νR and ns sterile fermion singlets s, leading to what we denote as ISS(nR, ns). In this framework, the generic mass Lagrangian for leptons is given in the flavour basis by. The ISS mechanism can be implemented by extending the SM particle content with nR RH neutrinos νR and ns sterile fermion singlets s, leading to what we denote as ISS(nR, ns).1 In this framework, the generic mass Lagrangian for leptons is given in the flavour basis by. We remark that the existence of a massless neutrino is presently not excluded by data In such case, there is only one physical Majorana phase and, the total number of physical parameters in the lepton sector is reduced to ten. It is clear that VL† Ws defines the mixing between the three active neutrinos and the nR + ns sterile states in the physical charged-lepton basis. Due to the additional fermion states, active-neutrino mixing is determined by the non-unitary matrix [66]. Where we used the first relation in eq (2.21) in order to write ηαβ solely in terms of the active-sterile mixing
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