Constraining axion–nucleon coupling constants from measurements of effective Casimir pressure by means of micromachined oscillator

Stronger constraints on the pseudoscalar coupling constants of an axion to a proton and a neutron are obtained from an indirect measurement of the effective Casimir pressure between two Au-coated plates by means of micromechanical torsional oscillator. For this purpose, the additional effective pressure due to two-axion exchange is calculated. The role of boundary effects and the validity region of the proximity force approximation in application to forces of axion origin are determined. The obtained constraints are up to factors of 380 and 3.2 stronger than those found recently from other laboratory experiments and are relevant to axion masses from 10 - 3 to 15 eV.