To understand the mass hierarchy problem, it's essential to understand the basics of neutrinos and their behavior. Neutrinos are created when certain types of atomic nuclei undergo radioactive decay. They are extremely light, with masses that are typically measured in electronvolts (eV). However, neutrinos can also oscillate between different mass states, which means that their mass is not fixed and can vary over time.
There are three known mass states for neutrinos, which are denoted by the letters "e," "μ," and "τ." The "e" state is associated with electrons, the "μ" state with muons, and the "τ" state with tau particles. The problem is that we don't know which state corresponds to which mass.