Search for monopole-dipole interactions at the millimeter range

Monopole-dipole interactions involve scalar couplings between spin and massive particles. Such interactions violate P- and T-symmetry, and can be transmitted by axions. A ¹²⁹Xe-¹³¹Xe-Rb comagnetometer is used to measure the ratio of precession frequencies between the two Xe isotopes, and search for changes of the ratio correlated with the distance between a nonmagnetic BGO crystal and the atomic cell. The search is sensitive to the monopole-dipole coupling at the millimeter range. Currently the statistical error of the frequency ratio measurement has reached the level of 10^-7 with a bias field of 10 mG.