Decoupling Neutrino Magnetic Moment from Mass with $SU(2)_L$ Invariance

TL;DR

This paper introduces a new mechanism that allows neutrinos to have observable magnetic moments without inducing large neutrino masses, by leveraging $SU(2)_L$ invariance to decouple the two effects.

Contribution

The authors propose a novel $SU(2)_L$-based mechanism that naturally separates neutrino magnetic moments from mass generation, avoiding fine-tuning in Standard Model extensions.

Findings

Mechanism suppresses neutrino mass while allowing sizable magnetic moments.

Explicit UV completions demonstrate the feasibility of the proposed model.

Predicted magnetic moments are within current and future experimental sensitivities.

Abstract

Standard Model extensions that yield observable neutrino magnetic moments typically also induce large neutrino masses, incompatible with experimental limits. This tension motivates the search for mechanisms that naturally decouple magnetic moments from mass generation without requiring fine-tuning. In this letter, we propose a novel mechanism for generating Dirac and Majorana neutrino magnetic moment, in which the associated mass contribution is forbidden by $SU(2)_L$ invariance. By carefully selecting the $SU(2)_L$ representations connecting the neutrino to the loop diagram, we ensure that only the effective dipole operator involving the non-Abelian part of the photon -- the neutral $SU(2)_L$ gauge boson -- is generated. Crucially, the corresponding mass diagram, obtained by removing the external gauge boson leg, vanishes. We provide explicit UV completions that implement this mechanism and yield neutrino magnetic moments within the sensitivity of current and future experiments.