New two-sided bound on the isotropic Lorentz-violating parameter of modified Maxwell theory

There is a unique Lorentz-violating modification of the Maxwell theory of photons, which maintains gauge invariance, CPT, and renormalizability. Restricting the modified-Maxwell theory to the isotropic sector and adding a standard spin-1/2 Dirac particle p(+/-) with minimal coupling to the nonstandard photon gamma, the resulting modified-quantum-electrodynamics model involves a single dimensionless deformation parameter, kappa(tr). The exact tree-level decay rates for two processes have been calculated: vacuum Cherenkov radiation p(+/-)-> p(+/-)gamma for the case of positive kappa(tr) and photon decay gamma -> p(+)p(-) for the case of negative kappa(tr). 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 kappa(tr) is obtained, which improves by 8 orders of magnitude upon the best direct laboratory bound. The implications of this result are briefly discussed.