Question of Lorentz invariance in muon decay
  Possibilities to test the Lorentz invariance of the weak interaction in muon decay are considered. We derive the direction-dependent muon-decay rate with a general Lorentz-violating addition to the W-boson propagator. We discuss measurements of the directional and boost dependence of the Michel parameters and of the muon lifetime as a function of absolute velocity. The total muon-decay rate in the Lorentz-violating Standard Model Extension is addressed. Suggestions are made for dedicated (re)analyses of the pertinent data and for future experiments. 
J. P. Noordmans, C. J. G. Onderwater, H. W. Wilschut, and R. G. E.   Timmermans
hep-ph nucl-th