Extremely high energy cosmic neutrino beams initiate high energy particle cascades in the background of relic neutrinos from the Big Bang.

Shown below is the energy distribution of neutrinos in the cascade after propagation through 1 Gpc. The input spectra are electron neutrinos with energies of 10^23 and 4x10^21 eV respectively. The pions in the jets initiated by the quarks produced in the cascade are the main supplier of the bulk of the secondary particles n the lower energy region.