A search for an annual variation of a daily sidereal modulation of the frequency difference between co-located ${ }^{ 129}$Xe and ${ }^{ 3}$He 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 $10^{ -27}$ GeV.
Categories: physics.atom-ph hep-ex hep-ph physics.gen-ph