  The Pierre Auger (cosmic ray) Observatory provides a laboratory for studying
fundamental physics at energies far beyond those available at colliders. The
Observatory is sensitive not only to hadrons and photons, but can in principle
detect ultrahigh energy neutrinos in the cosmic radiation. Interestingly, it
may be possible to uncover new physics by analyzing characteristics of the
neutrino flux at the Earth. By comparing the rate for quasi-horizontal, deeply
penetrating air showers triggered by all types of neutrinos, with the rate for
slightly upgoing showers generated by Earth-skimming tau neutrinos, we
determine the ratio of events which would need to be detected in order to
signal the existence of new non-perturbative interactions beyond the TeV-scale
in which the final state energy is dominated by the hadronic component. We use
detailed Monte Carlo simulations to calculate the effects of interactions in
the Earth and in the atmosphere. We find that observation of 1 Earth-skimming
and 10 quasi-horizontal events would exclude the standard model at the 99%
confidence level. If new non-perturbative physics exists, a decade or so would
be required to find it in the most optimistic case of a neutrino flux at the
Waxman-Bahcall level and a neutrino-nucleon cross-section an order of magnitude
above the standard model prediction.
