neutrino mass hierarchy

File:Hierfig.pdf The problem of neutrino mass hierarchy is related to the fact that present experimental data on neutrino oscillations allow two possible classes of solutions.{{cite book|title=Introduction to particle and astroparticle physics (multimessenger astronomy and its particle physics foundations)|first1=Alessandro |last1=De Angelis |first2=Mario |last2=Pimenta |year=2018|publisher=Springer|isbn=978-3-319-78181-5|doi=10.1007/978-3-319-78181-5}}

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.{{citation

| title = NuFit: An updated global analysis of neutrino oscillation measurements

| url = http://www.nu-fit.org/

| accessdate = 2020-12-03

}}