ar X iv : h ep - p h / 99 10 31 6 v 1 1 3 O ct 1 99 9 Relic Neutrinos , Z – Bursts , and Cosmic Rays above 10 20 eV

The observation of cosmic–ray events above the Greisen–Kuzmin–Zatsepin (GZK) cutoff of EGZK ∼ 5×1019 eV challenges orthodox modeling. We discuss a possible solution which uses standard hot Big Bang cosmology and Standard Model (SM) particle physics augmented only by < ∼ eV neutrino masses as suggested by solar, atmospheric, and terrestrial neutrino detection. In this scheme, cosmic ray neutrinos from distant, highest–energy sources annihilate resonantly on the relic–neutrino background to produce Z–bosons. The highly–boosted (γZ ∼ 10) Z’s instantly decay, producing “Zbursts” of highly–collimated jets of hadrons and photons. The burst content includes, on average, twenty photons and two nucleons with super–GZK energies. We show that the probability for each neutrino with energy within a fraction ΓZ/MZ of the resonant value ER = 4 (eV/mν)× 10 eV to annihilate within the halo of our galactic supercluster is likely within an order of magnitude of 1%. Depending on the magnitude of the cosmic neutrino flux above 10 eV, this “local” rate for primary production may be high enough to produce the cosmic ray events observed above the GZK cutoff. Several tests of this Z–burst hypothesis for generating super–GZK events are presented, including (i) a new cutoff energy at ER; (ii) a large γ/p ratio for primaries near the upper end of the spectrum; (iii) directional pairing of events, and pointing to their cosmic sources; and (iv) a neutrino flux above the GZK cutoff energy which is possibly measurable directly in proposed > ∼ 10 ton cosmic–ray detectors. 1 VAND-TH-9-99, Invited talk presented at “Beyond the Desert 99”, ed. H. Klapdor– Kleingrothaus, Ringberg Castle, Tegernsee, Germany, June 6–12, 1999