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

Abstract

Cosmogenic neutrinos are created when ultra-high-energy cosmic rays (UHECRs) interact with extragalactic photon backgrounds. In general, the expected flux of these cosmogenic neutrinos depends on multiple parameters, describing the sources and propagation of UHECRs. In our recent paper [1], we show that a 'sweet spot' occurs at a neutrino energy of E ν ∼ 1 EeV. At that energy the flux mainly depends on two parameters, the source evolution and the fraction of protons in UHECRs at Earth for E p 30 EeV. Therefore, with current upper limits on the cosmogenic neutrino flux at E ν ∼ 1 EeV and assuming a certain source class, a constraint on the composition of UHECRs can be obtained. This constraint is independent of hadronic interaction models and indicates that the combination of a large proton fraction and a strong source evolution is disfavored. Upcoming neutrino experiments will be able to constrain the fraction of protons in UHECRs even further, and for any realistic model for the evolution of UHECR sources.

A New View on Auger Data and Cosmogenic Neutrinos in Light of Different Nuclear Disintegration and Air-shower Models

“…The low neutrino fluxes are partly related to our choice of generic source model, which leads to fits with low maximal rigidity. Other scenarios are possible in which a small fraction of the UHECR flux originates from proton accelerators that reach GZK energies (van Vliet et al 2019). These protons would copiously produce cosmogenic neutrinos off the denser CMB and peak at higher energies, while the majority of UHECRs would have a heavier mass composition, in line with current observations.…”

“…Another impacting assumption is that of single dominant source population. The complexity can be increased by accounting for an additional proton component with higher rigidity (van Vliet et al 2019) or even by a detailed modelling of individual nearby sources (Eichmann et al 2018). This however also vastly increases the degrees of freedom, making a global fit of all free parameters unfeasible given the current statistics of the UHECR data.…”

“…Since neutrinos travel unimpeded through the IGM, the density of UHECRs for z > 1 has an impact on their flux. As a consequence, the cosmogenic neutrino flux can be used to constrain the cosmological source evolution (Ahlers et al 2009;Gelmini et al 2012;Aloisio et al 2015;Heinze et al 2016;Romero-Wolf & Ave 2017;Alves Batista et al 2019b;Møller et al 2018;Das et al 2018;Wittkowski & Kampert 2018;van Vliet et al 2019).…”

Improved Limits on Cosmogenic Fluxes from Ultra-High Energy Cosmic Rays

“…One can notice that future neutrino observatories are capable of detecting cosmogenic neutrino emission in the EeV range if the contribution of protons above 50 EeV is at least 10%. This is consistent with earlier results [4,44].…”