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, ALPs) with masses $10^{-2}~{\rm eV} \lesssim m \lesssim 10^{4}~{\rm eV}$ are improved by a factor of $\sim 100$. The first atomic-scale constraints on several exotic velocity-dependent dipole-dipole interactions are established as well. 
Filip Ficek, Derek F. Jackson Kimball, Mikhail Kozlov, Nathan Leefer,   Szymon Pustelny, Dmitry Budker
physics.atom-ph hep-ph