The problem of neutrino mass hierarchy is related to the fact that present experimental data on neutrino oscillations allow two possible classes of solutions.[1]
In the first class, called Normal Hierarchy (NH) or Normal Ordering (NO), the two lightest mass eigenstates have a small mass difference, of the order of 10 meV, while the third eigenstate has a mass about 50 meV higher. In the Inverted Hierarchy (IH), also called Inverted Ordering (IO), the lightest mass eigenstate is followed by a doublet of higher mass eigenstates about 50 meV heavier, being again of about 10 meV the mass difference in the doublet. Present data slightly prefer the NO.[2]
- ^ De Angelis, Alessandro; Pimenta, Mario (2018). Introduction to particle and astroparticle physics (multimessenger astronomy and its particle physics foundations). Springer. doi:10.1007/978-3-319-78181-5. ISBN 978-3-319-78181-5.
- ^ NuFit: An updated global analysis of neutrino oscillation measurements, retrieved 2020-12-03