Majorana neutrino masses in the scenario of gauge-Higgs unification

TL;DR

This paper explores mechanisms for generating small Majorana neutrino masses within gauge-Higgs unification models, highlighting the challenges of gauge invariance and proposing seesaw-based solutions involving higher-dimensional operators.

Contribution

It introduces novel gauge-invariant operators and mechanisms, including a double seesaw, for Majorana neutrino mass generation in gauge-Higgs unification scenarios.

Findings

Seesaw mechanism involves admixture of Type I and III.

Higher-dimensional operators enable Majorana mass generation.

Masses are generated via a double seesaw mechanism.

Abstract

In this paper we consider possible mechanisms to generate small Majorana neutrino masses for active neutrinos in the scenario of gauge-Higgs unification, a candidate for physics beyond the standard model. We stress that it is non-trivial to find a gauge-invariant operator, responsible for the Majorana masses, which is the counterpart of the well-known SU(2)$_L \times$ U(1)$_Y$ invariant higher-mass-dimensional ($d = 5$) operator. As the first possibility we discuss the seesaw mechanism by assigning leptonic fields to the adjoint representation of the gauge group, so that a $d = 5$ gauge-invariant operator can be formed. It turns out that the mechanism leading to the small Majorana masses is the admixture of the Type I and Type III seesaw mechanisms. As the second possibility, we consider the case where the relevant operator has $d = 7$, by introducing a matter scalar belonging to the fundamental representation of the gauge group. Reflecting the fact that the mass dimension of the operator is higher than usually expected, the Majorana masses are generated by a "double seesaw mechanism."