Neutrino clustering and the Z-burst model

Abstract

The possibility that the observed Ultra High Energy Cosmic Rays are generated by high energy neutrinos creating "Z-bursts" in resonant interactions with the background neutrinos has been proposed, but there are difficulties in generating enough events with reasonable incident neutrino fluxes. We point out that this difficulty is overcome if the background neutrinos have coalesced into "neutrino clouds" --- a possibility previously suggested by some of us in another context. The limitations that this mechanism for the generation of UHECRs places on the high energy neutrino flux, on the masses of the background neutrinos and the characteristics of the neutrino clouds are spelled out.Comment: 13 pages and 3 figures. Contributed to the XX International Symposium on Lepton and Photon Interactions at High Energies, Rome, July 2001, and to the International Europhysics Conference on High Energy Physics, Budapest, July 2001. Preprint numbers added, misprints correcte