Francis M. Pipkin Award Talk: Lorentz and CPT Symmetry Tests with Atomic Co-Magnetometers

Deviations from Lorentz symmetry naturally arise in many extensions of the Standard Model seeking to include quantum gravity effects. One of the common features of such models is coupling to a particle spin that violates local Lorentz invariance. We have developed a co-magnetometer using spin-polarized K and $^3$He atoms to perform a sensitive search for such effects. The co-magnetometer measures the difference between spin interactions of electrons and $^3$He nuclei, thereby canceling the signals from ordinary magnetic fields. The apparatus is rotated every 20 seconds to search for a spatial anisotropy. We have placed a limit on neutron interaction energy with a background Lorentz-violating field below $3.7\times 10^{-33}$ GeV, improving the previous limit by a factor of 30. Because of close connection between CPT and Lorentz symmetry, this measurement also represents the most stringent test of CPT for a fermion. One can also test Lorentz-violating theories that do not break CPT symmetry by using particles with a spin greater than 1/2 to search for a tensor spin anisotropy. We are currently using a co-magnetometer with $^{21}$Ne atoms to perform such a search.