Bound on Lorentz andCPTViolating Boost Effects for the Neutron

A search for an annual variation of a daily sidereal modulation of the frequency difference between colocated ${}^{129}\mathrm{X}\mathrm{e}$ and ${}^3\mathrm{H}\mathrm{e}$ Zeeman masers sets a stringent limit on boost-dependent Lorentz and $CPT$ violation involving the neutron, consistent with no effect at the level of 150 nHz. In the framework of the general standard-model extension, the present result provides the first clean test for the fermion sector of the symmetry of spacetime under boost transformations at a level of ${10}^{-27}\mathrm{G}\mathrm{e}\mathrm{V}$.