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The latest observations of extensive air showers (EAS) induced byultra-high-energy cosmic rays (UHECR) appear to indicate, prima facie, atransition to heavy primaries at the highest energies. However, thisinterpretation, which is based on extrapolations of the Standard Model (SM) toultra-LHC energies, is strained from both astrophysical and particlephenomenology perspectives. We consider the alternative that after some energythreshold, the first collision of the primary in the atmosphere results in astate, the decay of which leads to a considerably increased shower particlemultiplicity, so that light-primary EAS appear heavy-like. We show that aminimal implementation of such a model yields predictions for the average EASdepth 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, wepredict that: (a) the center-of-momentum (CM) energy threshold for the effectis 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) theincrease in multiplicity compared to the SM prediction grows with CM energyroughly as $\sim E_{\rm CM}$; (d) the cosmic-ray composition at the highestenergies is light. Remarkably, if the latter is confirmed electromagneticallythis would necessitate the existence of new physics by these energies.

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