Cosmogenic neutrinos challenge the proton dip model
The origin and composition of ultra-high-energy cosmic rays (UHECRs) remain a mystery. The proton dip model describes the shape of the cosmic ray spectrum above 10^9 GeV by the effect of a pure proton spectrum propagating through the cosmic microwave background. In these interactions secondary neutrinos are produced, which peak around 10^9 GeV.
We fit the recent UHECR spectrum measurements from the Telescope Array experiment under the assumption of pure proton composition, as assumed by the proton dip model.
We present a a full scan of the three main physical model parameters of UHECR-injection: source redshift evolution, injected maximal proton energy and spectral power-law index. We discuss how the result qualitatively changes compared to earlier two-parameter fits in the literature: a mild preference for a maximal energy cutoff at the sources instead of the Greisen*Zatsepin*Kuzmin (GZK) cutoff, hard injection spectra, and strong source evolution.
We show that the predicted neutrino flux exceeds the IceCube limit for any parameter combination. As a result, the proton dip model is challenged at more than 95% C.L. This is strong evidence against the dip-model independent of mass composition measurements.