# Improved constraints on monopole-dipole interaction mediated by pseudo-scalar bosons

## TL;DR

This paper reports a new, more stringent upper limit on axion-mediated long-range spin-dependent forces, improving previous constraints by an order of magnitude and discussing future enhancements to probe CP-violation and QCD axion models.

## Contribution

The study provides the most restrictive upper limit to date on axion-like particle coupling constants in the specified range, using a novel experimental setup and measurement technique.

## Key findings

- Upper limit on coupling strength improved by an order of magnitude.
- Measured variations in magnetization linked to axion-mediated interactions.
- Future plans aim to enhance sensitivity to CP-violating parameters.

## Abstract

We present a more stringent upper limit on long-range axion-mediated forces obtained by the QUAX-g$_p$g$_s$ experiment, located at the INFN - Laboratori Nazionali di Legnaro. We investigate the possible coupling between the electron spins of a paramagnetic GSO crystal and unpolarized nucleons of lead disks by measuring variations of GSO magnetization with a dc-SQUID magnetometer. Such an induced magnetization can be interpreted as the effect of a long-range spin dependent interaction mediated by axions or Axion Like Particles (ALPs). The corresponding coupling strength is proportional to the CP violating term $g_p^eg_s^N$, i.e. the product of the pseudoscalar and scalar coupling constants of electron and nucleon, respectively. Previous upper limits are improved by one order of magnitude, namely $g_p^eg_s^N/(\hbar c) \le 4.3\times10^{-30}$ at 95% confidence level, in the interaction range $10^{-3}$ m $<\lambda_a<0.2$ m. We eventually discuss our plans to improve the QUAX-g$_p$g$_s$ sensitivity by a few orders of magnitude, which will allow us to investigate the $\vartheta\simeq 10^{-10}$ range of CP-violating parameter and test some QCD axion models.