= Mixing of exactly massless neutrinos is removable
If all <neutrinos> are exactly massless, arbitrary unitary rotations among them preserve both their <kinetic terms> and their zero mass matrix. After diagonalizing the charged-lepton <Yukawa matrix>, choose the neutrino basis to coincide with that rotation. The resulting <weak charged current> has no physical lepton mixing matrix. With nondegenerate neutrino masses this extra basis freedom is lost, permitting observable <neutrino oscillations> and lepton mixing.
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