A new bound on Lorentz violation based on the absence of vacuum Cherenkov radiation in ultra-high energy air showers

In extensive air showers induced by ultra-high energy (UHE) cosmic rays, secondary particles are produced with energies far above those accessible other means. These extreme can be used to search for new physics. We study the effects of isotropic, nonbirefringent Lorentz violation in photon sector. case a velocity smaller than maximum attainable standard Dirac fermions, vacuum Cherenkov radiation becomes possible. Implementing this Lorentz-violating effect shower simulations, significant reduction calculated average atmospheric depth $\left$ is obtained. Based on and its shower-to-shower fluctuations $\sigma(X_\text{max})$, bound derived which improves previous one factor 2. This first such based absence from fundamental (electrons positrons) showers. Options further improvements discussed.