Implications of the KM3NeT Ultrahigh-energy Event on Neutrino Self-interactions

TL;DR

This work investigates neutrino self-interactions (νSI) mediated by a light scalar and their imprint on astrophysical neutrino spectra, focusing on the ultrahigh-energy KM3NeT event KM3-230213A. By modeling a τ-flavored scalar mediator with $m_φ$ around the 100 MeV scale and solving a transport equation for propagation through the cosmic neutrino background, the authors perform joint fits to IceCube HE data and KM3NeT/UHE data, finding that νSI can modestly alleviate the tension between the KM3NeT event and IceCube non-detections and yielding competitive 95% CL constraints on $g_{ττ}$ at $m_φ \sim 100$ MeV, in some cases surpassing collider bounds. Bayesian model comparison shows no strong overall preference for νSI, though certain single-power-law fits exhibit mild support; future IceCube-Gen2 measurements are expected to significantly extend sensitivity, potentially addressing cosmological tensions such as the Hubble and neutrino-mass discrepancies. Overall, the paper provides a robust astrophysical probe of νSI in a mediator-mass window that is challenging for terrestrial experiments, highlighting the complementary role of high-energy neutrino astronomy in constraining BSM neutrino interactions.

Abstract

Neutrino self-interactions ($ν$SI) mediated by light bosonic particles can produce characteristic spectral dips in astrophysical neutrino fluxes, thereby altering the expected energy spectrum. The high-energy astrophysical neutrino spectrum has been extensively used to probe $ν$SI models through these distinctive features. The recent detection of the ultrahigh-energy event KM3-230213A presents a new opportunity to explore $ν$SI phenomenology at extreme energies. In this work, we investigate two implications of this observation, assuming the event originates from a diffuse power-law spectrum. First, we find that $ν$SI-induced spectral distortions can mildly alleviate the tension between the KM3-230213A detection and the previous non-observation of PeV-scale neutrinos in IceCube data. Second, we derive the strongest constraints on the $τ$-flavored $ν$SI coupling strength for mediator masses around 100 MeV. Our analysis shows that neutrino telescopes can surpass existing collider bounds in this mass range. In the near future, IceCube-Gen2 is expected to significantly enhance $ν$SI sensitivity, including regions relevant to alleviating the Hubble and neutrino mass tensions.

Figures (6)

• Figure 1: In both panels, solid colored lines show the fitted flux with $\nu$SI effects, while dashed lines represent the fits without $\nu$SI. The colored bands indicate $1\sigma$ uncertainties from joint fits with $\nu$SI. Black crosses represent the segmented flux from observations, and the blue cross denotes the KM3NeT event. The gray shaded region shows flux limits from various observations ANTARES:2024ihwPierreAuger:2023pjgIceCube:2018fhm, and the black dashed lines indicate projected IceCube-Gen2 reach IceCube-Gen2:2020qhaMeier:2024flg.
• Figure 2: The 95% CL constraints on $\nu$SI mediator masses and couplings from the UHE+NST seg. (green shaded) and UHE+ESTES seg. (orange shaded) joint fits. The dashed lines show our projected exclusion for IceCube-Gen2 IceCube-Gen2:2020qhaMeier:2024flg based on the UHE+NST and UHE+ESTES $\nu$SI-Fit flux parameters.The gray region marks exist exclusions from BBN, $Z$ boson decays, and IceCube HESE Brdar:2020nbjEsteban:2021tub. The region between the black dashed lines highlights the parameter space that may resolve the Hubble tension, and the dot-dashed lines indicate space that could address the neutrino mass tension Poudou:2025qcx.
• Figure 3: In the left panel of the plot, we show the constraints on $\nu$SI coupling derived without the KM3-230213A event, in dashed colored lines, while the original constraints are in shaded regions. In the right panel, the ratios of the constraints with and without the KM3-230213A event are shown in colored lines.
• Figure 4: The posterior distribution of the KM3NeT event number expectation value with energy in the 1 $\sigma$ error bar range. The results for $\nu$SI and the SPL fits of the UHE + ESTES data set are shown in blue red shaded, respectively.
• Figure 5: We show the $90$ % and $95$ % favored region in posterior of the flux intensities and the flux indices. The results for the $\nu$SI and the SPL fits of the UHE + ESTES data set are shown in blue shaded region and red shaded region. The stars mark the best fit points, respectively.