Parameterizing Majorana Neutrino Couplings in the Higgs Sector

TL;DR

This paper presents a detailed parameterization of Majorana neutrino couplings within a Higgs sector that includes a doublet and a triplet, highlighting differences from charged fermions and Dirac neutrinos, and discusses implications for neutrino mass models.

Contribution

It introduces a comprehensive parameterization of neutrino-Higgs couplings in models with Majorana neutrinos, including right-handed neutrinos and scalar singlets, and compares it to existing frameworks like Casas-Ibarra.

Findings

Yukawa couplings depend on neutrino masses and mixing matrix elements.

In the mass basis, neutrino Yukawa couplings are generally non-diagonal.

Parameter constraints allow full exploration of physically distinguishable parameters.

Abstract

Nonzero masses for the active neutrinos - regardless of their nature or origin - arise only after electroweak symmetry breaking. We discuss the parameterization of neutrino couplings to a Higgs sector consisting of one SU(2)_L scalar doublet and one SU(2)_L scalar triplet, and allow for right-handed neutrinos whose Majorana mass parameters arise from the vacuum expectation value of a Standard Model scalar singlet. If the neutrinos are Majorana fermions, all Yukawa couplings can be expressed as functions of the neutrino mass eigenvalues and a subset of the elements of the neutrino mixing matrix. In the mass basis, the Yukawa couplings are, in general, not diagonal. This is to be contrasted to the case of charged-fermions or Dirac neutrinos, where couplings to the Higgs-boson are diagonal in the mass basis and proportional only to the fermion masses. Nonetheless, all physically distinguishable parameters can be reached if all neutrino masses are constrained to be positive, all mixing angles constrained to lie in the first quadrant (theta in [0,pi/2]), and all Majorana phases to lie in the first two quadrants (phi in [0,pi]), as long as all Dirac phases vary within the entire unit circle (delta in [0,2pi}). We discuss several concrete examples and comment on the Casas-Ibarra parameterization for the neutrino Yukawa couplings in the case of the type-I Seesaw Lagrangian.