  We have performed a calculation in the parton-model approximation of a
nonstandard decay process in isotropic modified Maxwell theory coupled to
standard Dirac theory, with a single Lorentz-violating parameter $\kappa$ in
the photonic sector. Previous calculations of this process (vacuum Cherenkov
radiation by a proton for the theory with $\kappa>0$) were performed for
pointlike particles, and an upper bound on $\kappa$ at the $10^{-19}$ level was
obtained from experimental data on ultra-high-energy cosmic rays. The
parton-model results change the decay rate by about an order of magnitude but
give the same upper bound on $\kappa$ because the threshold energy is
unchanged. The previous point-particle calculation of photon decay into an
electron-positron pair for the theory with $\kappa<0$ remains valid, and
experimental results on cosmic gamma rays provide a lower bound on $\kappa$ at
the $-10^{-15}$ level.
