Lepton mixing matrix ($U_{\alpha i}$)

The matrix relating weak-interaction neutrino flavour fields to mass eigenfields, ${\nu }_{\alpha L}={\sum }_i{U}_{\alpha i}{\nu }_{iL}$. In the standard three-active-neutrino description it is unitary. Oscillation probabilities involve products of entries and mass-squared differences; lepton-number-violating amplitudes can depend on additional Majorana phases through products such as $U_{ei}^2{m}_i$.