New Limits on Anomalous Spin-Spin Interactions

We report the results of a new search for long-range spin-dependent interactions using a Rb-Ne21 atomic comagnetometer and a rotatable electron spin source based on a SmCo5 magnet with an iron flux return. By looking for signal correlations with the orientation of the spin source we set new constraints on the product of the pseudoscalar electron and neutron couplings gpegpn/ℏc<1.7×10-14 and on the product of their axial couplings gAegAn/ℏc<5×10-42 to a new particle with a mass of less than about 1 μeV. Our measurements improve by about 2 orders of magnitude previous constraints on such spin-dependent interactions.