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

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 $ν$SI in the high-mass regime, potentially offering crucial insights into the connections between the neutrino sector and dark sector.

Figures (5)

• Figure 1: Mean-free-path estimate of the $\nu$SI strength relevant to UHE neutrino propagation. The black dashed lines are obtained using Eq. \ref{['eq:L-mean-min']}, and the blue region is obtained by further imposing a finite range of $E_{\nu}$.
• Figure 2: Neutrino energy spectrum without $\nu$SI calculated using our semi-analytic framework, which is consistent with the result from computationally intensive simulations CRPropa:2016qdtLeal:2025eou within the theoretical uncertainty characterized by the difference between the two solid or dashed lines. We show the results for our benchmark cases, $\Gamma=2.5$, $E_{\max}=250$ EeV, with $m=0$ and 3 being optimistic and pessimistic scenarios, respectively; the same conclusion applies to results for other parameter values.
• Figure 3: UHE neutrino fluxes modified by $\nu$SI. The left and right panels assume flavor universal and $\nu_{\tau}$-philic couplings, respectively. The upper panels present the flux spectrum, whereas the lower panels show the ratio of the modified flux to the standard one with free propagation.
• Figure 4: Left: direction-averaged effective area of GRAND for UHE tau neutrino detection. Right: our calculated event rates of UHE neutrinos at GRAND with ten years of exposure. We also use two histograms to show our energy binning.
• Figure 5: (Main result of the paper.) Our projected sensitivities on $\nu$SI from UHE neutrinos absorbed by relativistic CNB, i.e., under the "widening the resonance" scenario, along with results from previous works. Red solid (dashed) curves: our sensitivities using the GRAND UHE neutrino detector, assuming optimistic (pessimistic) fluxes. Blue curves: previous results from UHE neutrinos absorbed by non-relativistic CNB with optimistic (solid) and pessimistic (dashed) fluxes Leal:2025eou. Purple dashed curve: earlier results from HE neutrinos absorbed by relativistic CNB and detected by IceCube-Gen2 (dashed) Esteban:2021tub. The gray shaded regions are those excluded by $Z$ boson invisible decay Brdar:2020nbj, BBN Blinov:2019gcj, rare meson decay (universal coupling only) Berryman:2018ogk, and IceCube HE neutrino observation ($\nu_\tau$-philic coupling only) IceCube:2020wum.