  Nonbirefringent modified-Maxwell theory, coupled to standard Dirac particles,
involves nine dimensionless parameters, which can be bounded by the inferred
absence of vacuum Cherenkov radiation for ultrahigh-energy cosmic rays
(UHECRs). With selected UHECR events, two-sided bounds on the eight
nonisotropic parameters are obtained at the 10^{-18} level, together with an
improved one-sided bound on the single isotropic parameter at the 10^{-19}
level.

High Energy Physics - Phenomenology