Extra dimensions and strong neutrino-nucleon interactions above 1019 eV: breaking the GZK barrier

Cosmic ray events above 1020 eV are on the verge of confronting fundamental particle physics. The neutrino is the only candidate primary among established particles capable of crossing 100 Mpc intergalactic distances unimpeded. The magnitude of νN cross sections indicated by events, plus consistency with the Standard Model at low-energy, point to new physics of massive spin-2 exchange. In models based on extra dimensions, we find that the νN cross section rises to typical hadronic values of between 1 and 100 mb at energies above 1020 eV. Our calculations take into account constraints of unitarity. We conclude that air-showers observed with energies above 1019 eV are consistent with neutrino primaries and extra-dimension models. An upper bound of 1–10 TeV on the mass scale at which graviton exchange becomes strong in current Kaluza–Klein models follows.