Limit on Lorentz and CPT violation of the proton using a hydrogen maser

We present a new measurement constraining Lorentz and $\mathrm{CPT}$ violation of the proton using a hydrogen maser double resonance technique. A search for hydrogen Zeeman frequency variations with a period of the sidereal day (23.93 h) sets a limit on violation of Lorentz and $\mathrm{CPT}$ symmetry of the proton at the ${10}^{\ensuremath{-}27}$ GeV level, independent of nuclear model uncertainty, which improves significantly on previous bounds.