The singly-charged scalar singlet as the origin of neutrino masses

We consider the generation of neutrino masses via a singly-charged scalar singlet. Under general assumptions we identify two distinct structures for the neutrino mass matrix. This yields a constraint for the antisymmetric Yukawa coupling of the singly-charged scalar singlet to two left-handed lepton doublets, irrespective of how the breaking of lepton-number conservation is achieved. The constraint disfavours large hierarchies among the Yukawa couplings. We study the implications for the phenomenology of lepton-flavour universality, measurements of the W-boson mass, flavour violation in the charged-lepton sector and decays of the singly-charged scalar singlet. We also discuss the parameter space that can address the Cabibbo Angle Anomaly.

Automated summary

We investigate the generation of neutrino masses using a singly-charged scalar singlet and identify two distinct structures for the neutrino mass matrix under general assumptions. This leads to a constraint on the antisymmetric Yukawa coupling of the scalar singlet, excluding large hierarchies among the Yukawa couplings. Our study explores the implications for various aspects of lepton phenomenology and discusses the parameter space relevant to addressing the Cabibbo Angle Anomaly.