Matter changes the effective mixing angle and level splitting of propagating neutrinos. The electron-flavour charged current potential is relative to the other active flavours. In the two-flavour Hamiltonian, equality of its two diagonal entries gives the displayed resonance condition. Adiabatic evolution through a varying density can therefore cause a large flavour change even when the local vacuum mixing angle is smaller. The equality alone does not guarantee adiabatic propagation.
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