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Cosmogenic neutrinos are created when ultra-high-energy cosmic rays (UHECRs)interact with extragalactic photon backgrounds. In general, the expected fluxof these cosmogenic neutrinos depends on multiple parameters, describing thesources and propagation of UHECRs. In our recent paper (van Vliet et al. 2019),we show that a `sweet spot` occurs at a neutrino energy of $E_{\nu} \sim 1$EeV. At that energy the flux mainly depends on two parameters, the sourceevolution and the fraction of protons in UHECRs at Earth for $E_p \gtrsim 30$EeV. Therefore, with current upper limits on the cosmogenic neutrino flux at$E_{\nu} \sim 1$ EeV and assuming a certain source class, a constraint on thecomposition of UHECRs can be obtained. This constraint is independent ofhadronic interaction models and indicates that the combination of a largeproton fraction and a strong source evolution is disfavored. Upcoming neutrinoexperiments will be able to constrain the fraction of protons in UHECRs evenfurther, and for any realistic model for the evolution of UHECR sources.

Current constraints from cosmogenic neutrinos on the fraction of protons in UHECRs