Bounds on Lorentz violation from observations of extensive air showers

Due to their extremely high particle energies (up to 10^20 eV), cosmic rays are ideally suited to search for New Physics, for example violations of Lorentz invariance. In the following, isotropic, nonbirefringent Lorentz violation in the photon sector is considered, specializing to the case of a photon velocity larger than the maximum attainable velocity of standard Dirac fermions. Up to now, Earth-based bounds on this type of Lorentz violation have been determined from observations of TeV gamma rays. A novel approach to test Lorentz invariance with improved sensitivity is based on the measurement of extensive air showers induced by cosmic-ray particles in the Earth’s atmosphere. Lorentz-violating processes (e.g. photon decay into an electron-positron pair) can have a large impact on the longitudinal development of air showers. Compared to standard physics, these Lorentz-violating processes reduce the average atmospheric depth of the shower maximum Xmax for air showers with primary energies above 10^18 eV by an amount that is significantly larger than the average resolution of current air shower experiments. This can be exploited to obtain new Earth-based bounds on this type of Lorentz violation.