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Contributions to Pion Decay from Lorentz Violation in the Weak Sector

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.