Probing low- x QCD with cosmic neutrinos at the Pierre Auger Observatory

The sources of the observed ultrahigh energy cosmic rays must also generate ultrahigh energy neutrinos. Deep inelastic scattering of these neutrinos with nucleons on Earth probe center-of-mass energies √s~100 TeV, well beyond those attainable at terrestrial colliders. By comparing the rates for two classes of observable events, any departure from the benchmark (unscreened perturbative QCD) neutrino-nucleon cross section can be constrained. Using the projected sensitivity of the Pierre Auger Observatory to quasihorizontal showers and Earth-skimming tau neutrinos, we show that a ``super-Auger'' detector can thus provide an unique probe of strong interaction dynamics.