Searching for Lorentz Violation in the Ground State of Hydrogen
  A theoretical analysis is made of experiments in hydrogen and antihydrogen that test CPT. We show that these experiments also provide sensitive tests of Lorentz symmetry for protons and electrons. Both 1S-2S transitions and Zeeman hyperfine splittings in the ground state are examined. We find that sensitivity to CPT and Lorentz violation at the level of $10^{-27}$ GeV is attainable. 
Robert Bluhm, V. Alan Kostelecky, and Neil Russell