Neutrino mixing sum rules and the Littlest Seesaw

Abstract

In this work, we revisit neutrino mixing sum rules arising from discrete symmetries, and the class of Littlest Seesaw neutrino models. These theoretical models offer predictions for the leptonic CP phase and mixing angles, and correlations among them, which can be tested in forthcoming neutrino experiments. In particular, we study the solar neutrino mixing sum rules, arising from charged lepton corrections to Tri-bimaximal (TB), Bi-maximal (BM), Golden Ratios (GRs), and Hexagonal (HEX) neutrino mixing, and atmospheric neutrino mixing sum rules, arising from preserving one of the columns of these types of mixing, for example, the first or second column of the TB mixing matrix (TM1 or TM2), and confront them with an up-to-date global fit of the neutrino oscillation data. We show that some mixing sum rules, for example, an atmospheric neutrino mixing sum rule arising from a version of neutrino Golden Ratio mixing (GRa1), are already excluded at $3\sigma$, and determine the remaining models allowed by the data. We also consider the highly predictive Littlest Seesaw models, which are a special case of tri-maximal mixing models (TM1), and discuss their prospects.

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