Constraints on exotic spin-dependent interactions between electrons from helium fine-structure spectroscopy

Agreement between theoretical calculations of atomic structure and spectroscopic measurements is used to constrain possible contribution of exotic spin-dependent interactions between electrons to the energy differences between states in helium-4. In particular, constraints on dipole-dipole interactions associated with the exchange of pseudoscalar bosons (such as axions or axion-like particles) with masses 10^-2m10⁴eV are improved by a factor of ~100. The first atomic-scale constraints on several exotic velocity-dependent dipole-dipole interactions are established as well.