What do the highest-energy cosmic-ray data suggest about possible new physics around 50 TeV?

Abstract: The latest observations of extensive air showers (EAS) induced by ultra-high-energy cosmic rays (UHECR) appear to indicate, prima facie, a transition to heavy primaries at the highest energies. However, this interpretation, which is based on extrapolations of the Standard Model (SM) to ultra-LHC energies, is strained from both astrophysical and particle phenomenology perspectives. We consider the alternative that after some energy threshold, the first collision of the primary in the atmosphere results in a state, the decay of which leads to a considerably increased shower particle multiplicity, so that light-primary EAS appear heavy-like. We show that a minimal implementation of such a model yields predictions for the average EAS depth and shower-to-shower fluctuations that are consistent with each other, and an excellent fit to Auger data. If such an effect indeed takes place, we predict that: (a) the center-of-momentum (CM) energy threshold for the effect is of order 50 TeV; (b) the probability with which the effect occurs is high, and it will be detected easily by next-generation accelerators; (c) the increase in multiplicity compared to the SM prediction grows with CM energy roughly as $\sim E_{\rm CM}$; (d) the cosmic-ray composition at the highest energies is light. Remarkably, if the latter is confirmed electromagnetically this would necessitate the existence of new physics by these energies.