Observed reactor and atmospheric neutrino oscillation mixing values appear to be related to the neutrino scale ratio ∆m 2 sol /∆ 2 AT M in a way that suggest that the neutrino mass matrix can be expanded as a power series by using this ratio as the smallness parameter. This approach provides a simple and natural way to expose the inner hierarchies among neutrino mass terms, which amounts to also explain the solar oscillation mixing as well as solar oscillation scale. We explore a class of mass matrix textures that realize this scenario, for both normal and inverted neutrino mass hierarchies, as well as CP violation and their stability under renormalization scaling.
A sterile neutrino in the $$3+1$$
3
+
1
scheme, where the sterile accounts for neutrino anomalies not explained solely by the weak active neutrinos, arises as a natural source for the breaking of the $$\mu -\tau $$
μ
-
τ
symmetry suggested by oscillation neutrino data. We explore the predictions for the Dirac CP phases in this scenario, with and without sterile neutrino decay, and show that current limits on $$\delta _{CP}$$
δ
CP
suggest a normal hierarchy and a lightest neutrino scale below 0.1 eV as the most plausible explanation for that, when Majorana phases are null. Other Dirac phases turn out to be non zero as well.
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