Constraining interactions mediated by axion-like particles with ultracold neutrons

TL;DR

This study sets new constraints on short-range spin-dependent interactions potentially mediated by axion-like particles by measuring neutron and mercury spin precession frequencies, improving previous limits significantly in a specific interaction range.

Contribution

It provides the first precise measurement constraining axion-like particle mediated interactions using ultracold neutrons and mercury atoms, enhancing existing bounds in the 10^{-6} to 10^{-4} meter range.

Findings

Established new upper limits on axion-like particle interactions.

Improved constraints by up to two orders of magnitude in the specified range.

Confirmed model-independent bounds consistent with previous experiments.

Abstract

We report a new limit on a possible short range spin-dependent interaction from the precise measurement of the ratio of Larmor precession frequencies of stored ultracold neutrons and $^{199}$Hg atoms confined in the same volume. The measurement was performed in a $\sim$1$\mu$ T vertical magnetic holding field with the apparatus searching for a permanent electric dipole moment of the neutron at the Paul Scherrer Institute. A possible coupling between freely precessing polarized neutron spins and unpolarized nucleons of the wall material can be investigated by searching for a tiny change of the precession frequencies of neutron and mercury spins. Such a frequency change can be interpreted as a consequence of a short range spin-dependent interaction that could possibly be mediated by axions or axion-like particles. The interaction strength is proportional to the CP violating product of scalar and pseudoscalar coupling constants $g_Sg_P$. Our result confirms limits from complementary experiments with spin-polarized nuclei in a model-independent way. Limits from other neutron experiments are improved by up to two orders of magnitude in the interaction range of $10^{-6}<\lambda<10^{-4}$ m.