Neutrinos: Majorana or Dirac or in-between?
Bhupal Dev (Washington University, St. Louis/USA)

Neutrinos are the least known particles in the Standard Model. In particular, whether they are Dirac or Majorana particles is an important open question. Theoretically, it is also possible that they are pseudo-Dirac, which are fundamentally Majorana fermions, but essentially act like Dirac fermions in most experimental settings, due to extremely small active-sterile mass splitting. We will discuss how such tiny values of mass splitting can be accessed via oscillations over astrophysical baselines. We use the recent multi-messenger observations of high-energy neutrino sources to probe hitherto unexplored values of mass splitting, which improve the reach of terrestrial experiments by more than a billion. Finally, we will present an intriguing hint for nonzero active-sterile mass splitting in the recent IceCube data.