Lorentz violation in the weak sector would affect the beta-decay lifetimes of
pions. The decay amplitude may be rendered anisotropic, but only an isotropic
violation of boost invariance can affect the net lifetime in the center of mass
frame. However, since the rest frames of the pions that produce the NuMI
neutrino beam at Fermilab vary with the rotation of the Earth, it is possible
to constrain anisotropic Lorentz violation using prior analyses of sidereal
variations in the event rate at the MINOS near detector. The resulting bounds
on weak-sector Lorentz violation are at the 10^(-4) level, a substantial
improvement over previous results. The highly relativistic character of the
pions involved is responsible for the improvement.Comment: 16 page
