The Milky Way is a Laboratory for New Ultra-long-baseline Neutrino Physics

The IceCube Neutrino Observatory recently published evidence for diffuse neutrino emission from the Galactic Plane at $4.5\sigma$ significance. This new source of astrophysical neutrinos provides an exciting laboratory for probing the nature of neutrino masses. In particular, extremely small mass splittings, such as those predicted by quasi-Dirac neutrino mass models, and finite neutrino lifetimes from neutrino decays, would induce effects on the spectra and flavor ratios of neutrinos with TeV-scale energies traversing kiloparsec-scale baselines. Using TANDEM, an upcoming three dimensional galactic neutrino emission model, we explore the sensitivity of IceCube and KM3NeT/ARCA to these ultra-long-baseline phenomena. We find that a combined analysis would be sensitive to quasi-Dirac mass splittings $10^{-14.0}~\mathrm{eV^2} \lesssim \delta m^2 \lesssim 10^{11.6}~\mathrm{eV^2}$ and neutrino lifetimes $m / \tau \gtrsim 10^{-14.1}~\mathrm{eV^2}$ at $> 1\sigma$, both regions constituting as-yet unexplored parameter space. Our results demonstrate the potential that astrophysical neutrino sources and global neutrino telescope networks have in probing new regions of exotic neutrino mass models.