Implications of the penetration depth of ultrahigh-energy cosmic rays on physics at 100 TeV

The simple interpretation of Pierre Auger Observatory ultrahigh energy cosmic rays (UHECRs) penetration depth measurements suggests a transition at the energy range 1.1-35×1018 eV from protons to heavier nuclei. A detailed comparison of this data with air shower simulations reveals strong restrictions on the amount of light nuclei (protons and He) in the observed flux. We find a robust upper bound on the observed proton fraction of the UHECR flux and we rule out a composition dominated by protons and He. Acceleration and propagation effects lead to an observed composition that is different from the one at the source. Using a simple toy model that takes into account these effects, we show that the observations require an extreme metallicity at the sources with metals to protons mass ratio of 1â̂1, a ratio that is larger by a factor of a hundred than the solar abundance. This composition imposes an almost impossible constraint on all current astrophysical models for UHECR accelerators. This m4ay provide a first hint toward new physics that emerges at ∼100 TeV and leads to a larger proton cross section at these energies.