Widen the Resonance at Ultra-High Energies: Novel Probes of Neutrino Self-interactions in the High-Mass Regime

TL;DR

This paper introduces a new method to detect neutrino self-interactions at ultra-high energies by exploiting a widened resonance effect, enabling future telescopes to probe previously inaccessible parameter spaces and deepen understanding of neutrino and dark sectors.

Contribution

It proposes a novel resonance-widening technique for UHE neutrino scattering, providing a semi-analytic framework and extending the sensitivity of future neutrino telescopes to high-mass mediator regimes.

Findings

Widened resonance enhances sensitivity to neutrino self-interactions.

Future telescopes can probe mediator masses from MeV to GeV.

Coupling sensitivities reach down to g ~ 10^{-3}, surpassing current bounds.

Abstract

Neutrino self-interaction beyond the Standard Model is well motivated by the nonzero masses of neutrinos, which are the only known particles guaranteed to have new physics. Cosmic messengers, especially neutrinos, play a central role in probing new physics, as they provide experimental conditions far beyond the reach of laboratories and serve as the link between laboratory fundamental-physics discoveries and their roles in the Universe, where many new physics motivations originate. In this work, we propose a novel probe of neutrino self-interactions through ultra-high-energy neutrinos scattering off the cosmic neutrino background when the lightest neutrino species remains relativistic today. This allows us to ``Widen the Resonance'' of such scattering. Meanwhile, we also provide a semi-analytic framework for cosmogenic UHE neutrino production, avoiding computationally intensive simulations and yielding results precise enough for BSM studies. The widened resonance enables future ultrahigh-energy neutrino telescopes, in particular GRAND, to probe mediator masses from MeV to GeV, reaching couplings down to $g \sim 10^{-3}$ -- up to two orders of magnitude beyond current bounds. Our results enhance the discovery potential of $\nu$SI in the high-mass regime, potentially offering crucial insights into the connections between the neutrino sector and dark sector.