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

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)(e)g(s)(N), i.e. the product of the pseudoscalar and scalar coupling constants of electron and nucleon, respectively. Our upper limit is more constraining than previous ones in the interaction range 0.01 m < lambda(a) < 0.2 m, with a best result on g(p)(e)g(s)(N)/(hc) of 4.3 x 10(-30) at 95% confidence level in the interval 0.01 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 nu similar or equal to 10(-10) range of CP-violating parameter and test some QCD axion models. (C) 2017 The Author(s). Published by Elsevier B.V.