Neutrino transition dipole moments in light of the KM3NeT event

TL;DR

This work uses the ultra-high-energy neutrino KM3NeT-230213A to probe the transition magnetic dipole portal between active and sterile neutrinos by evaluating Earth-induced attenuation. By computing the up-scattering cross section in the DIS regime and integrating over the Earth’s density along the neutrino path, the authors derive bounds on the portal coupling $\mu$ as a function of the sterile mass $m_s$, requiring the optical depth $\tau$ to remain modest. The resulting exclusion contours show that for $m_s \gtrsim 1$ TeV, KM3NeT provides constraints stronger than existing IceCube and collider bounds, effectively extending the reach of neutrino-dipole searches into the multi-TeV regime. The study demonstrates the potential of individual ultra-high-energy neutrino events to inform physics beyond the Standard Model and underscores the value of future detections for refining these limits.

Abstract

In 2023, KM3NeT observed the highest energy neutrino seen to date. The most probable estimate places the energy of the neutrino at an extreme value of 220 PeV, and energies up to 2.6 EeV are contained in the $90\%$ probability interval. Even though the source of the signal cannot be ascertained with the available data, this observation already opens unique possibilities to test new physics. Here, we study the implications for the transition magnetic dipole portal connecting active and sterile neutrinos. We remain agnostic regarding the origin of the neutrino and focus on local effects. Concretely, we study the contribution of an efficient active to sterile transitions to the opacity seen on the path to the detector at Earth. We find that the ultra-high-energy neutrino event does not add relevant information for sterile neutrinos with masses less than a TeV. However, the observation becomes important at higher masses and can provide the leading constraint in that regime.

Figures (3)

• Figure 1: Depth of the detector $D$ along the neutrino track in units of m.w.e. as a function of the angle with respect to the nominal direction (green solid line). The underwater relief provided by KM3NeT:2025npi has been taken and $\rho_{rock}=2.6\,\rho_{water}$ has been considered. Similar results have been included for comparison purposes, taking into account a medium consisting solely of water (blue dashed line) or rock (red dashed line).
• Figure 2: The nucleon-averaged neutrino cross sections of conversion to a sterile neutrino per unit of $\mu$ squared as a function of $m_s$ for the extremal and favoured values of the $E_\nu$ at a confidence level of $90\%$. The collision target here is fixed as ${\rm H}_2{\rm O}$ for demonstration.
• Figure 3: Bounds for $\mu$ for the KM3NeT event as a function of $m_s$. Each color band collects the upper limits for different trajectories i.e. $-2.2^\circ\leq\theta\leq2.2^\circ$ being the angle with respect to the nominal direction (solid line). The red region corresponds to $E_\nu=72$ PeV, the blue region to $E_\nu=220$ PeV, and the green region to $E_\nu=2.6$ EeV. The golden line based on IceCube observations is taken from Huang:2022pce, the purple solid, dashed and dot-dashed line based on LEP ($d_\gamma$ and $d_{\gamma Z}$) and LHC, respectively, are taken from Magill:2018jla.