Detecting Cosmic Rays of the Highest Energies

The highest energies are defined, at a given time, by our detector capabilities. Thus the highest energies will be the just measured or soon to be measured energies. To-day, the frontier in cosmic rays reaches 10 eV. This represents a macroscopic energy, namely 16 J, transported by a microscopic object. For charged particles, the highest energy events seem to necessitate some New Physics, beyond the present understanding of astrophysics and particle physics. Concerning neutrinos, apart from the low energy ones coming from the explosion of SN1987A, only neutrinos of terrestrial origin have been detected so far. It is a challenge to detect neutrinos which may be witnesses of the most violent phenomena in the Universe. With their very small probability of interactions and their neutral nature, they can come unaffected from the most distant parts of the Universe. Theoretical predictions give two regions of interest: from 10 eV to 10 eV for the contribution of AGN’s, and from 10 eV to 10 eV for GZK neutrinos, the ones created but the high energy cosmic rays interacting with the CMB. Moreover, the study of high energy neutrinos may be used, not only to find sources, but also to constrain some properties of these elusive particles. For example, limits on oscillations can be achieved for δm unreachable otherwise, and one can dream of searching for cosmic neutrinos through the Z resonant absorption.