Photon decay in ultrahigh-energy air showers: Stringent bound on Lorentz violation

In extensive air showers induced by ultra-high-energy (UHE) cosmic rays, secondary photons are expected to be produced at energies far above those accessible other means. It has been shown that the decay of such photons, as possible in certain theories allowing Lorentz violation, can lead significant changes shower development. Based on observations average depth maximum $\left$, a stringent bound violation placed previous work. Here we include shower-to-shower fluctuations $\sigma(X_\text{max})$ an additional observable. The combined comparison $\left$ and allows much stricter test UHE improving factor 50.