On the energy of the protons producing the very high-energy astrophysical neutrinos

TL;DR

This paper analyzes the energy distribution of protons responsible for producing high-energy astrophysical neutrinos, highlighting how the proton energies can significantly exceed simple estimates and depend on various spectral assumptions.

Contribution

It provides a detailed calculation of proton energy distributions for neutrino production via $pp$ and $p ext{ extgamma}$ interactions, considering spectral effects and threshold behaviors.

Findings

Proton energies can be much larger than 20 times the neutrino energy.

The proton energy distribution is sensitive to the assumed proton and photon spectra.

Threshold effects in $p ext{ extgamma}$ interactions cause notable increases in proton energies at lower neutrino energies.

Abstract

We study the distribution of the energy of the protons that can produce an astrophysical neutrino with a given observed energy, in the TeV--PeV range, both through $pp$ or $p\gamma$ interactions. Due to the increasing multiplicity of the pion production at high center of mass energies, the resulting average proton energies can be much larger than the often used approximate value $E_p\simeq 20E_\nu$. Also the threshold of the $p\gamma$ process can lead to a pronounced increase in the values of $\langle E_p\rangle /E_\nu$ for decreasing neutrino energies. The results depend sensitively on the assumed proton spectrum, since steeper spectra give less weight to the lower energy neutrino tail resulting from the decays of the abundant low-energy pions. In the $p\gamma$ scenarios they also depend sensitively on the spectrum of the target photons. The results are in particular relevant to relate possible characteristics of the neutrino spectrum to those of the corresponding cosmic rays that generated them. We also discuss the associated production of gamma rays at the sources.