Ultra high energy neutrinos: absorption, thermal effects and signatures

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 ∼ 10 −2 eV), thermal effects are important for ultra high energy neutrino sources at z > ∼ 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 ∼ 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 ∼ 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 ∼ 10 − 100.1 After neutrino decoupling, T is actually defined by Eq. (2.5).

Cosmic absorption of ultra high energy particles

“…The resonance produces a dip around E r /(1 + z min ) ≃ 1.7 × 10 21 (m ν /0.08 eV) −1 eV, where z min ≃ 30. Additional broadening of the resonance, due to thermal effect, is discussed in detail in [22]. At higher energies the corresponding redshift is z ≃ 87.…”

“…Below we compute the horizon due to interaction of UHE neutrinos with cosmic neutrino background (CνB). Following [22] we assume CνB neutrinos are in their mass states. The cross-section is composed of two parts [22].…”

“…Following [22] we assume CνB neutrinos are in their mass states. The cross-section is composed of two parts [22]. The resonant neutrino annihilation occurs in the s-channel:…”

“…We assume that neutrino are non-relativistic even at sufficiently high redshift, which is a good approximation for z < 10 2 for m ν = 0.08 eV/c 2 . Effects of non-zero momentum on the neutrino annihilation cross-section are studied in [22,23]. Using the number density of relic neutrinos n CνB ≃ 112 cm −3 and the non-resonant cross-section in the high energy limit one can estimate the horizon for UHE neutrinos at highest energies.…”

Probing BSM neutrino physics with flavor and spectral distortions: Prospects for future high-energy neutrino telescopes

“…The neutrinos could instead be relatively warm, either from having a larger temperature than naively expected or by having mass smaller than or comparable to the T CνB . Then in this case thermal broadening would allow for a significant scattering over a much wider range of energies [103,104]. Note that for the case displayed in Fig.…”