Neutrino mass hierarchy

The neutrino mass hierarchy is an open question in particle physics. Neutrinos are the lightest particles in the Standard Model which are known to have mass. Experimental measurements of neutrino oscillations can only determine the differences between neutrino masses, not the absolute masses themselves. Therefore, there are two possible arrangements consistent with current data: the normal ordering (NO) and the inverted ordering (IO), also called the normal hierarchy (NH) and inverted hierarchy (IH). Establishing the mass hierarchy is an active research topic, with several ongoing experiments investigating it.