Ultra High Energy Neutrinos: Absorption, Thermal Effects and Signatures

TL;DR

This paper investigates how ultra high energy neutrinos are absorbed by the cosmic neutrino background, considering thermal effects, and explores implications for distant astrophysical sources and neutrino physics.

Contribution

It provides a detailed analysis of thermal effects on neutrino absorption and applies findings to models of ultra high energy neutrino sources, highlighting the impact on observable spectra.

Findings

Thermal effects significantly influence neutrino absorption at high redshifts.

Suppression dips in neutrino transmission are limited to two due to thermal effects.

Broad suppression valleys in neutrino spectra can indicate sources beyond redshift 10.

Abstract

We study absorption of ultra high energy neutrinos by the cosmic neutrino background, with full inclusion of the effect of the thermal distribution of the background on the resonant annihilation channel. For a hierarchical neutrino mass spectrum (with at least one neutrino with mass below $\sim 10^{-2}$ eV), thermal effects are important for ultra high energy neutrino sources at $z \gtrsim 16$. The neutrino transmission probability shows no more than two separate suppression dips since the two lightest mass eigenstates contribute as a single species when thermal effects are included. Results are applied to a number of models of ultra high energy neutrino emission. Suppression effects are strong for sources that extend beyond $z \sim 10$, which can be realized for certain top down scenarios, such as superheavy dark matter decays, cosmic strings and cosmic necklaces. For these, a broad suppression valley should affect the neutrino spectrum at least in the energy interval $10^{12} - 10^{13}$ GeV -- which therefore is disfavored for ultra high energy neutrino searches -- with only a mild dependence on the neutrino mass spectrum and hierarchy. The observation of absorption effects would indicate a population of sources beyond $z \sim 10$, and favor top-down mechanisms; it would also be an interesting probe of the physics of the relic neutrino background in the unexplored redshift interval $z \sim 10 -100$.