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

The latest observations of extensive air showers (EAS) induced by ultra-high-energy cosmic rays (UHECR) appear to indicate 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. The alternative is that some new physical effect affects EAS above some energy threshold. Here, we quantify the phenomenological constraints encoded in the latest Auger cosmic-ray data for any such new phenomenon. We consider the possibility 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 early in the development of the shower, so that light-primary EAS appear heavy-like. We show that a minimal, generic 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 this is indeed the effect underlying the phenomenology revealed by Auger at the highest energies, we predict that: (a) the center-of-momentum (CM) energy threshold for the effect lies between 14 and 60 TeV, with most likely value at 45 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 for a collision of similar CM energy) grows with CM energy as $E_{\rm CM}^{1.14 \pm 0.07}$; (d) the cosmic-ray composition at the highest energies is light. Remarkably, if the latter is confirmed electromagnetically (e.g. through studies of primary propagation in the Milky Way magnetic field) then this would necessitate the existence of new physics by these energies.