New limits on long-range spin-dependent forces mediated by axion-like particles

I will describe recent results of our searches for new long-range spin-dependent forces using atomic co-magnetometers. In one experiment we used two 250 kg movable lead weights to search for a CP-violating spin-mass interaction with ³He nuclear spins. In another experiment we used a rotatable SmCo₅ permanent magnet to search for non-magnetic spin-spin interactions between electron and ²¹Ne nuclear spins. Both experiments use optically-pumped alkali-metal vapor to polarize the nuclear spins, readout their spin precession signals, and cancel ordinary magnetic fields acting on the nuclear spins. We have improved prior limits on anomalous nuclear spin dependent forces mediated by particles lighter than about 1 μeV. I will also briefly describe our current efforts to develop next-generation atomic co-magnetometers for precision measurements of nuclear spin-dependent interactions.