Large Neutrino Magnetic Moments in the Light of Recent Experiments

TL;DR

This paper explores how large neutrino magnetic moments suggested by XENON1T data can be compatible with small neutrino masses through new leptonic symmetries and models, including horizontal symmetries and medium-dependent masses.

Contribution

It introduces new leptonic symmetry models, such as $SU(2)_H$ and $SU(3)_H$, to reconcile large neutrino magnetic moments with small neutrino masses, and analyzes their experimental implications.

Findings

Large transition magnetic moments $ o$ potential explanation for XENON1T excess.

New symmetry models can suppress neutrino masses despite large magnetic moments.

Medium-dependent neutrino masses can evade astrophysical constraints.

Abstract

The excess in electron recoil events reported recently by the XENON1T experiment may be interpreted as evidence for a sizable transition magnetic moment $\mu_{\nu_e\nu_\mu}$ of Majorana neutrinos. We show the consistency of this scenario when a single component transition magnetic moment takes values $\mu_{\nu_e\nu_\mu} \in(1.65 - 3.42) \times 10^{-11} \mu_B$. Such a large value typically leads to unacceptably large neutrino masses. In this paper we show that new leptonic symmetries can solve this problem and demonstrate this with several examples. We first revive and then propose a simplified model based on $SU(2)_H$ horizontal symmetry. Owing to the difference in their Lorentz structures, in the $SU(2)_H$ symmetric limit, $m_\nu$ vanishes while $\mu_{\nu_e\nu_\mu}$ is nonzero. Our simplified model is based on an approximate $SU(2)_H$, which we also generalize to a three family $SU(3)_H$-symmetry. Collider and low energy tests of these models are analyzed. We have also analyzed implications of the XENON1T data for the Zee model and its extensions which naturally generate a large $\mu_{\nu_e\nu_\mu}$ with suppressed $m_\nu$ via a spin symmetry mechanism, but found that the induced $\mu_{\nu_e\nu_\mu}$ is not large enough to explain recent data. Finally, we suggest a mechanism to evade stringent astrophysical limits on neutrino magnetic moments arising from stellar evolution by inducing a medium-dependent mass for the neutrino.