Impact of coherent scattering on relic neutrinos boosted by cosmic rays

TL;DR

This paper investigates how ultra-high-energy cosmic rays can boost relic neutrinos through coherent scattering, potentially explaining high-energy neutrino observations and constraining relic neutrino overdensities.

Contribution

It provides the first detailed calculation of boosted relic neutrino flux considering both coherent and incoherent scatterings, and constrains relic neutrino overdensity using current experimental data.

Findings

Relic neutrino flux peaks at ~200 PeV energy.

Constraints on relic neutrino overdensity down to ~10^8.

Potential explanation for KM3NeT high-energy neutrino events.

Abstract

Ultra-high-energy cosmic rays (UHECR) scattering off the cosmic relic neutrino background have recently gained renewed interest in the literature. Current data suggest that (UHECR) are predominantly made of heavy nuclei. Similar to the coherent elastic neutrino-nucleus scattering (CE$\nu$NS) observed at low-energy neutrino experiments, the cross section of heavy nucleus scattering off relic neutrinos will be coherently enhanced since the energy of relic neutrinos can reach $\sim O(10)$ MeV in the rest frame of the UHECR. We calculate the diffuse flux of relic neutrinos boosted by UHECR after taking into account the contributions from both coherent and incoherent scatterings. Using current data from IceCube and Pierre Auger Observatory, we place constraints on the overdensity of relic neutrinos down to $\sim 10^8$. Since the flux of boosted relic neutrinos peaks at an energy of $\sim 200\, \text{PeV}$, we also entertain the possibility to explain the recently observed KM3NeT event with boosted relic neutrinos from UHECR.