Does the 220 PeV Event at KM3NeT Point to New Physics?

Abstract

The KM3NeT collaboration recently reported the observation of KM3-230213A, a neutrino event with an energy exceeding 100 PeV, more than an order of magnitude higher than the most energetic neutrino in IceCube's catalog. Given its longer data-taking period and larger effective area relative to KM3NeT, IceCube should have observed events around that energy. This tension has recently been quantified to lie between $2σ$ and $3.5σ$, depending on the neutrino source. A $\mathscr{O}(100)$ PeV neutrino detected at KM3NeT has traversed approximately $147$ km of rock and sea en route to the detector, whereas neutrinos arriving from the same location in the sky would have only traveled through about $14$ km of ice before reaching IceCube. We use this difference in propagation distance to address the tension between KM3NeT and IceCube. Specifically, we consider a scenario in which the source emits sterile neutrinos that partially convert to active neutrinos through oscillations. We scrutinize two such realizations, one where a new physics matter potential induces a resonance in sterile-to-active transitions and another one where off-diagonal neutrino non-standard interactions are employed. In both cases, sterile-to-active neutrino oscillations become relevant at length scales of $\sim100$ km, resulting in increased active neutrino flux near the KM3NeT detector, alleviating the tension between KM3NeT and IceCube. Overall, we propose the exciting possibility that neutrino telescopes may have started detecting new physics.

Figures (6)

• Figure 1: Topography of the vicinity of the KM3NeT and IceCube detectors in the nominal direction of an incoming $\mathcal{O}(100)$ PeV neutrino, as determined by the KM3NeT collaboration. We observe that the path through rock and sea for KM3NeT is an order of magnitude longer than the path through ice en route to the IceCube detector.
• Figure 2: Sterile-to-active neutrino oscillation probability, $P_{s\mu}$, is shown in the parameter space of the sterile neutrino mass $m_s$ and interaction strength $-\epsilon_{ss}$ for both KM3NeT and IceCube. The vacuum mixing angle is fixed to $\theta=10^{-2}$. Deeper shades of blue and red correspond to higher oscillation probabilities at KM3NeT and IceCube, respectively.
• Figure 3: Sterile-to-active neutrino oscillation probability, $P_{s\mu}$, is shown as a function of interaction strength $-\epsilon_{ss}$ for both KM3NeT and IceCube, for a sterile neutrino mass of $m_s=3$ keV and a vacuum mixing angle $\theta=10^{-2}$.
• Figure 4: The sterile-to-active neutrino oscillation probability, $P_{s\mu}$, in the scenario with off-diagonal non-standard neutrino interactions is shown for both KM3NeT and IceCube for $m_s=500$ eV.
• Figure S1: Energy dependence of the effective mixing parameters, for the benchmark points shown in the figure. Upper panels: Absolute value of the effective mixing angle in matter, $|\theta_m(E_\nu)|$, for both the resonant and non-resonant scenarios. For the resonant case, a clear enhancement appears between $E_\nu \sim 150$ and $300~\text{PeV}$, corresponding to the resonance region. For the non-resonant non-standard interaction scenario, the enhancement can be observed for $E_\nu \gtrsim \mathcal{O}(100)$ PeV. Lower panels: The effective mass-squared difference $|\Delta m_{\rm eff}^2(E_\nu)|$, for both scenarios.