Neutrino mass, ordering, nature and CP violation

6 problems

• What is the mass of the lightest neutrino?

  Open

  Oscillations measure only differences of neutrino masses. The overall scale must come from precise $\beta$ decay spectra or from how neutrinos affected the growth of cosmic structure.

• Do neutrino oscillations violate CP symmetry?

  Open

  If neutrinos and antineutrinos oscillate differently, leptons break the matter-antimatter mirror symmetry called CP. This may link to why the universe contains matter.

• Are neutrinos Majorana particles, their own antiparticles?

  Open

  If a neutrino is its own antiparticle, certain nuclei could undergo a double beta decay that emits no neutrinos at all. Its observation would show that lepton number is not conserved.

• Is the neutrino mass ordering normal or inverted?

  Open

  We know the mass differences between the three neutrinos but not whether the odd one out is heaviest (normal ordering) or lightest (inverted ordering).

• What mechanism generates neutrino masses?

  Open

  Neutrinos are at least a million times lighter than the electron. They could get mass like other particles but with a tiny coupling, or through a seesaw with very heavy partner particles.

• Is $\theta_{23}$ maximal, and if not, in which octant?

  Open

  Muon-type and tau-type neutrinos are mixed in nearly equal amounts, but it is unknown whether the mixing angle is exactly 45 degrees or slightly above or below it. Exactly 45 degrees would point to a hidden symmetry between muons and taus.