# Cosmogenic neutrinos as a probe of the transition from Galactic to extragalactic cosmic rays

## TL;DR

This paper investigates how cosmogenic neutrino fluxes can distinguish between two main scenarios explaining the cosmic ray spectrum ankle, providing insights into cosmic ray origins and source evolution.

## Contribution

It calculates neutrino fluxes for both scenarios, identifying characteristic spectral features that can help determine the correct model of cosmic ray transition.

## Key findings

- Neutrino flux at ~10^{20} eV is higher in the ankle-transition scenario with high maximum source energies.
- A peak at ~10^{16} eV in the neutrino spectrum supports the proton-dip scenario if extragalactic protons contribute down to 10^{17} eV.
- Neutrino flux at ~10^{18} eV depends on the cosmological evolution of cosmic ray sources.

## Abstract

There are two promising scenarios that explain the ankle, which is a dip in the spectrum of cosmic rays at $\sim 10^{19}$ eV. A scenario interprets the ankle as the transition from Galactic to extragalactic cosmic rays ({\it ankle-transition scenario}), while the other is that the dip caused by pair production on the cosmic microwave background radiation ({\it proton-dip scenario}). In this paper, we consider whether cosmogenic neutrinos can be a clue to judge which scenario is favored. We calculated the fluxes of cosmogenic neutrinos following these scenarios with plausible physical parameter sets, and found several important features as follows. First of all, the neutrino flux at $\sim 10^{20}$ eV becomes much higher in the ankle-transition scenario as long as the maximum energy of the cosmic rays at sources is sufficiently high. On the other hand, the neutrino spectrum has a characteristic peak at $\sim 10^{16}$ eV in the proton-dip scenario on the condition that extragalactic protons significantly contribute to the observed cosmic rays down to $10^{17}$ eV. Thus, we conclude cosmogenic neutrinos should give us a clue to judge which scenario is favored, unless these features are masked by the neutrino background coming from possible, powerful neutrino sources such as AGNs and GRBs. We also found an interesting feature that the neutrino flux at $\sim 10^{18}$ eV depends only on the cosmological evolution of the cosmic ray sources. That means cosmogenic neutrinos with the energy bring us information on the cosmological evolution of the sources of ultra-high energy cosmic rays. Finally, we compare the fluxes of cosmogenic neutrinos with the expected sensitivity curves of several neutrino detectors, and conclude the detection of cosmogenic neutrinos in the near future is promising.