Floor of Cosmogenic Neutrino Fluxes above 1017 eV

Abstract

The search for neutrinos with energies greater than 10 ^17 eV is being actively pursued. Although normalization of the dominant neutrino flux is highly uncertain, a floor level is guaranteed by the interactions of extragalactic cosmic rays with milky Way gas. We estimate that this floor level gives an energy flux of ${E}^{2}{\phi }_{\nu }\,\simeq {10}^{-{13}_{-0.5}^{+0.5}}\,\mathrm{GeV}\,{\mathrm{cm}}^{-2}\,{\mathrm{sr}}^{-1}\,{{\rm{s}}}^{-1}$ at 10 ^18 eV, where uncertainties arise from the modeling of the gas distribution and the experimental determination of the mass composition of ultra–high-energy cosmic rays on Earth. Based on a minimal model of cosmic-ray production to explain the mass-discriminated energy spectra observed on Earth above 5 × 10 ^18 eV, we also present generic estimates of the neutrino fluxes expected from extragalactic production that generally exceed the aforementioned guaranteed floor. The prospects for detecting neutrinos above 10 ^18 eV remain challenging, however, unless proton acceleration to the highest energies is at play in a subdominant population of cosmic-ray sources or new physical phenomena are at work.

Keywords

• Astrophysical processes
• Cosmic rays
• Non-thermal radiation sources