# An Axion-Like Particle from an $SO(10)$ Seesaw with $U(1)_X$

## TL;DR

This paper explores an SO(10) GUT model where a residual anomalous U(1)_X symmetry leads to an ultralight axion-like particle, connecting neutrino mass generation with a low-energy pseudoscalar in a novel way.

## Contribution

It introduces a new mechanism linking the see-saw neutrino masses with an axion-like particle via the Stueckelberg mechanism in an SO(10) GUT framework.

## Key findings

- Predicts an ultralight axion-like particle around 10^{-20} eV.
- Shows the residual U(1)_X symmetry arises from incomplete neutrino decoupling.
- Provides a concrete realization within SO(10) models.

## Abstract

We investigate the decoupling of heavy right handed neutrinos in the context of an SO(10) GUT model, where a remnant anomalous symmetry is $U(1)_{X}$. In this model the see-saw mechanism which generates the neutrino masses is intertwined with the Stueckelberg mechanism, which leaves the CP-odd phase of a very heavy Higgs in the low energy spectrum as an axion-like particle. Such pseudoscalar is predicted to be ultralight, in the $10^{-20}$ eV mass range. In this scenario, the remnant anomalous $X$ symmetry of the particles of the Standard Model is interpreted as due to the incomplete decoupling of the right handed neutrino sector. We illustrate this scenario including its realisation in the context of SO(10).