Limit on Lorentz and CPT Violation of the Proton Using a Hydrogen Maser
  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. 
D. F. Phillips, M. A. Humphrey, E. M. Mattison, R. E. Stoner, R. F C.   Vessot and R. L. Walsworth