Maximal atmospheric neutrino mixing from texture zeros and quasi-degenerate neutrino masses

TL;DR

This paper demonstrates that specific texture-zero neutrino mass matrices naturally lead to near-maximal atmospheric mixing and CP violation in a quasi-degenerate mass spectrum, independent of solar and reactor angles.

Contribution

It identifies two special texture-zero cases that inherently produce maximal atmospheric mixing and CP violation in quasi-degenerate neutrino masses, regardless of other mixing angles.

Findings

Two texture-zero cases lead to near-maximal atmospheric mixing.

These cases produce a maximal CP-violating phase under certain conditions.

The properties hold independently of solar and reactor mixing angles.

Abstract

It is well-known that, in the basis where the charged-lepton mass matrix is diagonal, there are seven cases of two texture zeros in Majorana neutrino mass matrices that are compatible with all experimental data. We show that two of these cases, namely B3 and B4 in the classification of Frampton, Glashow and Marfatia, are special in the sense that they automatically lead to near-maximal atmospheric neutrino mixing in the limit of a quasi-degenerate neutrino mass spectrum. This property holds true irrespective of the values of the solar and reactor mixing angles because, for these two cases, in the limit of a quasi-degenerate spectrum, the second and third row of the lepton mixing matrix are, up to signs, approximately complex-conjugate to each other. Moreover, in the same limit the aforementioned cases also develop a maximal CP-violating CKM-type phase, provided the reactor mixing angle is not too small.