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

Understand

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.