Determining the fraction of cosmic-ray protons at ultrahigh energies with cosmogenic neutrinos

Cosmogenic neutrinos are produced when ultrahigh-energy cosmic rays (UHECRs) interact with cosmological photon fields. Limits on the diffuse flux of these neutrinos can be used to constrain the fraction of protons arriving at Earth with energies $E_p\gtrsim 30\mathrm{EeV}$, thereby providing bounds on the composition of UHECRs without fully relying on hadronic interaction models. We show to which extent current neutrino telescopes already constrain this fraction of protons and discuss the prospects for next-generation detectors to further constrain it. Additionally, we discuss the implications of these limits for several popular candidates for UHECR source classes.

Figure 1

Simulated single-flavor cosmogenic neutrino ($\nu +\overline{\nu }$) spectra [assuming a $({\nu }_e:{\nu }_{\mu }:{\nu }_{\tau })=(1:1:1)$ flavor ratio] for pure-proton scenarios with $m=3.0$ and $f=1.0$. The corresponding cosmic-ray curves are normalized to the Auger spectrum [43] at $E_0=10^{19.55}\mathrm{eV}$. For reference, we also show the IceCube 6-yr HESE data [44] and the Auger [45, 46] and IceCube [47] differential 90% C.L. upper limits for single-flavor neutrinos and half-energy-decade fluxes.

Figure 2

Observable fraction of protons $f$ at ultrahigh energies as a function of the source evolution parameter $m$. Three different single-flavor flux levels at a neutrino energy of $E_{\nu }=1\mathrm{EeV}$ are shown, corresponding roughly to the current sensitivity of IceCube and Auger (yellow), and upper (red) and lower (green) ranges for the expected sensitivity of ARA, ARIANNA and GRAND200k.