Limits on local Lorentz invariance in mercury and cesium

We report bounds on local Lorentz invariance (LLI) violation in Cs and Hg. The limits are obtained through the observation of the spin-precession frequencies of ¹⁹⁹Hg and ¹³³Cs atoms in their ground states as a function of the orientation of an applied magnetic field with respect to the fixed stars. We measure the amplitudes of the dipole couplings to a preferred direction in the equatorial plane to be 19(11) nHz for Hg and 9(5) μHz for Cs. The upper bounds established here improve upon previous bounds by about a factor of 4. The improvement is primarily due to mounting the apparatus on a rotating table. Bounds are established on several terms in the standard model extension including the first bounds on the spin couplings of the neutron and proton to the z direction: b∼_zⁿ<7×10^-30 GeV and b∼_z^p<7×10^-29 GeV.