Further swampland constraint on Dirac Neutrino

TL;DR

This paper explores how swampland conjectures impose additional constraints on Dirac neutrino masses and symmetries, suggesting specific discrete symmetry breakings and tighter upper bounds on neutrino masses.

Contribution

It demonstrates that applying swampland conjectures to circle compactifications with a $U(1)$ symmetry yields new constraints on neutrino properties and symmetry breaking patterns.

Findings

Neutrino mass must be below approximately 10 meV.

$U(1)$ symmetry must break to specific discrete groups depending on hierarchy.

Stronger upper bounds on neutrino masses are predicted.

Abstract

Recent studies on swampland conjectures (e.g. non-SUSY AdS conjecture or AdS distance conjecture) predict that the (lightest) neutrino must be Dirac and the mass must be cosmologically small $m < c \Lambda^{1/4}_{\text{cc}}\sim 10 \ \text{meV}$. The Dirac neutrino naturally accompanies a $U(1)$ symmetry that can be embedded in the anomaly-free $U(1)_{B-L}$ global symmetry of the standard model. We point out that the swampland conjectures applied to a circle compactification with the $U(1)$ symmetry twisting lead to further constraints. In particular, the $U(1)$ symmetry must be broken down to $\mathbb{Z}_4$,$\mathbb{Z}_8$ or $\mathbb{Z}_{10}$ in the case of normal hierarchy, and $\mathbb{Z}_4$ in the case of inverted hierarchy, providing evidence for the absence of continuous global symmetry in quantum gravity. We also predict a more constrained upper bound of the neutrino mass for each case.