High Energy Astrophysical Neutrinos: the Upper Bound is Robust

TL;DR

This paper clarifies the physical basis and robustness of the upper bound on high energy neutrino fluxes derived from cosmic ray observations, impacting future neutrino detection experiments.

Contribution

It demonstrates that the Waxman-Bahcall bound is robust and conservative, and cannot be easily evaded by various astrophysical scenarios.

Findings

The upper bound applies to neutrinos from both p,gamma and p-p(n) reactions.

The bound is valid for sources optically thin to high energy protons.

The Waxman-Bahcall bound overestimates the flux by a factor of about 5/tau.

Abstract

We elucidate the physical basis for the upper bound on high energy neutrino fluxes implied by the observed cosmic ray flux. We stress that the bound is valid for neutrinos produced either by p,gamma reactions or by p-p(n) reactions in sources which are optically thin for high energy protons to photo-meson and nucleon-meson interactions. We show that the upper bound is robust and conservative. The Waxman-Bahcall bound overestimates the most likely neutrino flux by a factor ~ 5/tau, for small optical depths tau. The upper limit cannot be plausibly evaded by invoking magnetic fields, optically thick AGNs, or large hidden fluxes of extragalactic protons. We describe the implications of the bound for future experiments including the AMANDA, ANTARES, Auger, ICECUBE, NESTOR, and OWL/AIRWATCH detectors.