New classes of systematic effects in gas spin comagnetometers.

Atomic comagnetometers are widely used in precision measurements searching for spin interactions beyond the standard model. We describe a new (3)He-(129)Xe comagnetometer probed by Rb atoms and use it to identify two general classes of systematic effects in gas comagnetometers, one associated with diffusion in second-order magnetic-field gradients and another due to temperature gradients. We also develop and confirm experimentally a general and practical approach for calculating spin relaxation and frequency shifts due to arbitrary magnetic-field gradients.