Relic neutrinos and Z-resonance mechanism for highest energy cosmic rays

The origin of the highest energy cosmic rays remains elusive. The decay of a superheavy particle (X) into an ultraenergetic neutrino which scatters fr om a relic neutrino or antineutrino at the Z-resonance has attractive featu res. Given the necessary X mass of 10(14)-10(15) GeV, the required lifetime , 10(15)-10(16) yr, renders model building a serious challenge, but three l ogical possibilities are considered: (1) X is a Higgs scalar in SU( 15) bel onging to high-rank representation, leading to power-enhanced lifetime; (2) a global X quantum number has exponentially suppressed symmetry breaking b y instantons; and (3) with additional space dimension(s), localization of X within the real-world brane leads to Gaussian decay suppression, the most efficient of the suppression mechanisms considered.