Exploring New Propagation Scales With Galactic Neutrinos

The recent observation of high-energy Galactic neutrinos by IceCube allows for searches of new physics affecting neutrino propagation on scales of $O(10^9-10^{15}),\mathrm{km/GeV}$ in distance over energy. We assess the sensitivity of upcoming measurements of Galactic neutrinos by IceCube and KM3NeT to such new phenomena. We focus on two scenarios: quasi-Dirac neutrinos and neutrino decays. In the quasi-Dirac scenario, we find that joint measurements by IceCube and KM3NeT are sensitive to the mass-squared differences $δm^2 \in \left(10^{-13.5}\mathrm{eV^2}, 10^{-11.9}\mathrm{eV^2}\right)$ at the $90%$ confidence level. For neutrino decays, the same measurements are sensitive to mass over lifetime ratios $m / τ> 10^{-12.3}~\mathrm{eV^2}$ at the same significance. Our results demonstrate that measurements of Galactic neutrinos by a global network of neutrino telescopes can probe signatures of neutrino mass models.

💡 Research Summary

This paper presents a comprehensive forecast for the sensitivity of next-generation neutrino telescopes, specifically IceCube and KM3NeT, to new physics affecting neutrino propagation, using the recently detected flux of high-energy Galactic neutrinos as a probe. The central premise is that Galactic neutrinos, traveling kiloparsec-scale distances across the Milky Way at TeV-PeV energies, access an unprecedented range of the ratio of propagation distance to energy (L/E) — approximately 10^9 to 10^15 km/GeV. This scale is far beyond the reach of terrestrial neutrino experiments like those studying solar or atmospheric neutrinos, opening a new window to phenomena that could be associated with the unknown mechanism of neutrino mass generation.

The analysis focuses on two concrete Beyond the Standard Model (BSM) scenarios: quasi-Dirac (QD) neutrinos and neutrino decay. In the QD scenario, neutrinos are pseudo-Dirac particles where the three known mass eigenstates split into pairs of nearly degenerate states with a tiny mass-squared difference δm². This leads to new, high-frequency oscillation patterns described by a survival probability proportional to cos²(δm² L / 4E). In the decay scenario, neutrinos are assumed to decay into invisible products, with a survival probability following an exponential decay law, exp(-α L/E), where α = m/τ is the mass-to-lifetime ratio.

The methodology involves detailed modeling of the Galactic neutrino signal. The production is modeled using the TANDEM framework, which provides a three-dimensional map of neutrino emission based on Galactic gas distributions and cosmic-ray fluxes. To incorporate BSM effects, this emission is integrated along each line of sight, weighted by the appropriate survival probability from the QD or decay model. A key insight is the resulting strong directional dependence: neutrinos coming from the Galactic center (longer average L) show spectral distortions at different energies compared to those from the anti-center or high Galactic latitudes (shorter average L), as illustrated in the paper’s figures.

The detection prospects at IceCube and KM3NeT are evaluated by considering their complementary strengths. IceCube, located at the South Pole, has primarily detected the Galactic signal using cascade events, which have good energy resolution but poor angular resolution. KM3NeT, in the Northern Hemisphere, will be ideally suited to detect Galactic neutrinos via track events (from νμ) with excellent angular resolution, as these neutrinos arrive through the Earth, filtering out atmospheric muon backgrounds. The analysis accounts for the distinct energy and angular smearing characteristics of cascade-like and track-like events.

The core result is a forecast of the joint sensitivity of IceCube and KM3NeT to the BSM parameters. For a common mass-squared splitting across all three neutrino families in the QD scenario, the combined data from both experiments is projected to be sensitive to δm² in the range