  We present a new measurement constraining Lorentz and 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
clean limit on violation of Lorentz and CPT symmetry of the proton at the
10^{-27} GeV level.

Atomic Physics;High Energy Physics - Experiment;High Energy Physics - Phenomenology;General Physics