Improved Limits on Spin-Mass Interactions

TL;DR

This paper reports a new experimental search that sets improved constraints on hypothetical long-range forces mediated by axion-like particles between unpolarized and polarized fermions, significantly tightening previous limits.

Contribution

The study introduces a novel experimental setup using large lead weights and a co-magnetometer to enhance sensitivity to spin-mass interactions, achieving an order of magnitude better constraints.

Findings

Placed new upper limits on scalar-pseudoscalar coupling products.

Improved constraints for axion-like particles with masses below 10^{-6} eV.

Demonstrated the effectiveness of the experimental method in probing new physics.

Abstract

Very light particles with CP-violating couplings to ordinary matter, such as axions or axion-like particles, can mediate long-range forces between polarized and unpolarized fermions. We describe a new experimental search for such forces between unpolarized nucleons in two 250 kg Pb weights and polarized neutrons and electrons in a $^3$He-K co-magnetometer located about 15 cm away. We place improved constrains on the products of scalar and pseudoscalar coupling constants, $g^n_p g^N_s < 4.2\times10^{-30}$ and $g^e_p g^N_s < 1.7\times10^{-30}$ (95% CL) for axion-like particle masses less than $10^{-6}$ eV, which represents an order of magnitude improvement over the best previous neutron laboratory limit.