Floor of cosmogenic neutrino fluxes above 1017~eV
Abstract
The search for neutrinos with energies greater than 1017~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 E2φ 10-13+0.5-0.5~GeV~cm-2~sr-1~s-1 at 1018~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 5×1018~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 1018~eV remain however challenging, unless proton acceleration to the highest energies is at play in a sub-dominant population of cosmic-ray sources or new physical phenomena are at work.
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