Weak Triplet Models of Neutrino Magnetic Moments

TL;DR

This paper investigates models where neutrino magnetic moments can be enhanced without proportionally increasing neutrino masses, focusing on weak-triplet Dirac fermion frameworks and highlighting the challenges of natural realization.

Contribution

It critically examines the feasibility of decoupling neutrino magnetic moments from their masses in weak-triplet Dirac fermion models, revealing the necessity of fine-tuning and the limitations in extended scenarios.

Findings

Minimal models allow decoupling but require fine-tuning.

Extended models link magnetic moments to neutrino masses, making enhancement difficult.

Natural enhancement of neutrino magnetic moments remains highly nontrivial.

Abstract

Experimental limits on neutrino magnetic moments remain several orders of magnitude above the predictions of the Standard Model; therefore, any future detection would provide unambiguous evidence for new physics. In models with Dirac neutrinos, however, mechanisms that enhance the magnetic moment typically generate excessively large neutrino masses. Recently, it has been argued that in frameworks where neutrinos mix with weak-triplet Dirac fermions, the magnetic moment can be decoupled from the neutrino mass. In this work, we revisit this possibility and show that sizable enhancements remain highly nontrivial to realize naturally. We demonstrate that, although the minimal realization allows the magnetic moment to be decoupled from the neutrino mass, obtaining an observable enhancement requires a delicate adjustment of the model parameters. Moreover, in extended scenarios, the decoupling no longer persists: the magnetic moment and neutrino mass become intrinsically linked, such that attempts to enhance the former inevitably induce large contributions to the latter.