# Axion-like Relics: New Constraints from Old Comagnetometer Data

## TL;DR

This paper uses old comagnetometer data to set new, stringent terrestrial constraints on ultra-light axion-like particles interacting with neutrons and electrons, offering a complementary approach to stellar cooling constraints.

## Contribution

It derives the most stringent terrestrial constraints to date on axion-like particles coupled to neutrons using existing comagnetometer data, and discusses future improvements for enhanced sensitivity.

## Key findings

- Old data constrains axion-neutron interactions more tightly than previous terrestrial experiments.
- Constraints are comparable to stellar cooling bounds, providing a complementary detection method.
- Future improvements could increase sensitivity by several orders of magnitude.

## Abstract

The noble-alkali comagnetometer, developed in recent years, has been shown to be a very accurate measuring device of anomalous magnetic-like fields. An ultra-light relic axion-like particle can source an anomalous field that permeates space, allowing for its detection by comagnetometers. Here we derive new constraints on relic axion-like particles interaction with neutrons and electrons from old comagnetometer data. We show that the decade-old experimental data place the most stringent terrestrial constraints to date on ultra-light axion-like particles coupled to neutrons. The constraints are comparable to those from stellar cooling, providing a complementary probe. Future planned improvements of comagnetometer measurements through altered geometry, constituent content and data analysis techniques could enhance the sensitivity to axion-like relics coupled to nucleons or electrons by many orders of magnitude.