We report on a search for a new, short-range, spin-dependent interaction
using a modified version of the experimental apparatus used to measure the
permanent neutron electric dipole moment at the Paul Scherrer Institute. This
interaction, which could be mediated by axion-like particles, concerned the
unpolarized nucleons (protons and neutrons) near the material surfaces of the
apparatus and polarized ultracold neutrons stored in vacuum. The dominant
systematic uncertainty resulting from magnetic-field gradients was controlled
to an unprecedented level of approximately 4 pT/cm using an array of
optically-pumped cesium vapor magnetometers and magnetic-field maps
independently recorded using a dedicated measurement device. No signature of a
theoretically predicted new interaction was found, and we set a new limit on
the product of the scalar and the pseudoscalar couplings gsgpλ2<8.3×10−28m2 (95% C.L.) in a range of 5μm<λ<25mm for the monopole-dipole interaction. This new result confirms
and improves our previous limit by a factor of 2.7 and provides the current
tightest limit obtained with free neutrons