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In the last years a general consensus has emerged on the use of ultra-high energy cosmic rays (UHECR) data as a powerful probe of the validity of special relativity. This applies in particular to the propagation of cosmic rays from their sources to Earth, responsible for energy suppressions due to pion photoproduction by UHE protons (the Greisen-Zatsepin Kuzmin limit) and photo disintegration of UHE nuclei (the Gerasimova-Rozental limit). A suppression in the flux of UHECRs at energies above 40 EeV -- as expected from both these interactions -- has been established experimentally beyond any doubt by current experiments. However, such an observation is still not conclusive on the origin of the suppression. In particular, data from the Pierre Auger Observatory can be interpreted in a scenario in which the suppression is due to the maximum acceleration energy at the sources rather than to interactions in the background radiation. In this scenario, UHECR data can no longer yield bounds on Lorentz invariance violations which increase the thresholds for interactions of nuclei on background photons, in particular through modification of the dispersion relations. Here we argue in turn that the study of UHECRs still represents an opportunity to test Lorentz invariance, by discussing the possibility of deriving limits on violation parameters from UHECR phenomena other than propagation. In particular we study the modifications of the shower development in the atmosphere due to the possible inhibition of the decay of unstable particles, especially neutral pions.

Future prospects of testing Lorentz invariance with UHECRs