PHYSICS WITH COSMIC NEUTRINOS, PEV TO ZEV

Neutrinos offer a particularly promising eye on the extreme Universe. Neutrinos are not attenuated by intervening radiation fields such as the Cosmic Microwave Background, and so they are messengers from the very distant and very young phase of the universe. Also, neutrinos are not deflected by cosmic magnetic fields, and so they should point to their sources. In addition, there are particle physics aspects of neutrinos which can be tested only with cosmic neutrino beams. After a brief overview of highest-energy cosmic ray data, and the present and proposed experiments which will perform neutrino astronomy, we discuss two particle physics aspects of neutrinos. They are possible long-lifetime decay of the neutrino, and a measurement of the neutrino-nucleon cross-section at a CMS energy orders of magnitude beyond what can be achieved with terrestrial accelerators. Measurement of an anomalously large neutrino cross-section would indicate new physics (e.g. low string-scale, extra dimensions, precocious unification), while a smaller than expected cross-section would reveal an aspect of QCD evolution. We then discuss aspects of neutrino-primary models for the extreme-energy (EE) cosmic ray data. Primary neutrinos in extant data are motivated by the directional clustering at EE reported by the AGASA experiment. We discuss the impact of the strongly-interacting neutrino hypothesis on lower-energy physics via dispersion relations, the statistical significance of AGASA directional clustering, and the possible relevance of the Z-burst mechanism for existing EE cosmic ray data.