2 Mass hierarchy

The rate at which neutrinos oscillate from one flavour to another (e.g. from electron neutrinos to muon or tau neutrinos) tell us the difference between the squares of the neutrino mass states: Δm_ij² = m_j² - m_i². However, for oscillations in vacuum, this rate does not tell us the sign of the mass difference, and thus existing experiments allow two possible orderings for the neutrino masses (left and right). This figure also illustrates how the flavour states, ν_χ, are mixed to make the mass states, ν_i, (red indicates an electron flavour state, blue a muon, and green a tau). The red bit in the m₃ mass state shows that the figure assumes θ₁₃ is non-zero.

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