Speaker
Description
Whether neutrinos are Dirac or Majorana remains one of the deepest open questions in particle physics. An especially compelling possibility is that they are quasi‑Dirac: each active neutrino paired with a nearly degenerate sterile partner, split by a tiny Majorana mass plausibly generated by Planck‑scale or string‑theoretic lepton‑number violation. The signature is an ultra‑long‑baseline active‑to‑sterile oscillation, far beyond the reach of terrestrial experiments but naturally probed by the cosmological distances traversed by astrophysical neutrinos. We summarize three complementary studies that exploit neutrino telescopes to test this scenario: one using IceCube's observation of the point source NGC 1068, one using the diffuse all‑sky astrophysical flux, and one forecasting the reach of IceCube and KM3NeT measurements of Galactic neutrinos. Together, they show that neutrino telescopes — across point‑source, diffuse, and Galactic channels — can probe the quasi‑Dirac hypothesis over a vast range of mass splittings inaccessible to any other experiment, opening a direct window on Planck‑scale physics through the oscillations of cosmic neutrinos.
