The highest energy cosmic rays and new particle physics

It has been argued that the observations of cosmic particles with energies in excess of 108 TeV represent a puzzle, the solution of which requires new astrophysics or new particle physics. We show that the latter is unlikely given the general physics and observational constraints that any particle physics candidate must satisfy. We argue that the scale associated with a new particle physics threshold must be of order 1 GeV, not TeV and above, in order to resolve the problem. In most cases such new physics should have been revealed by accelerator experiments. In particular, we examine the possibility that the highest energy cosmic rays are initiated by non-standard interactions of neutrinos in the atmosphere. We show that proposals in this direction either violate S-wave unitarity, or fall short of producing a sizeable effect by several orders of magnitude.