There is a unique Lorentz-violating modification of the Maxwell theory of photons, which maintains gauge invariance, , and renormalizability. Restricting the modified-Maxwell theory to the isotropic sector and adding a standard spin- Dirac particle with minimal coupling to the nonstandard photon , the resulting modified-quantum-electrodynamics model involves a single dimensionless “deformation parameter,” . The exact tree-level decay rates for two processes have been calculated: vacuum Cherenkov radiation for the case of positive and photon decay for the case of negative . From the inferred absence of these decays for a particular high-quality ultrahigh-energy-cosmic-ray event detected at the Pierre Auger Observatory and a well-established excess of TeV gamma-ray events observed by the High Energy Stereoscopic System telescopes, a two-sided bound on is obtained, which improves by 8 orders of magnitude upon the best direct laboratory bound. The implications of this result are briefly discussed.

• Received 22 September 2008

DOI:https://doi.org/10.1103/PhysRevD.78.085026