  If Lorentz and CPT violation exist, they could affect the decays of scalar
and pseudoscalar particles. For a decay into a fermion and an antifermion (not
necessarily of the same mass), both the total decay rate and the outgoing
particle distribution may be modified, through interference between the
conventional decay mechanism and a separate Lorentz-violating mechanism. The
modifications are sensitive to forms of Lorentz violation that are otherwise
rather difficult to study, since at tree level they do not affect particle
propagation, but only interaction vertices. Using existing experimental data on
charged pion decay, it is possible to constrain three parameters in the
modified pion-muon-neutrino coupling at better than the $10^{-9}$ level; these
are the first bounds on these quantities.
