= MSW effect
{c}
{title2=$2E\sqrt2G_FN_e=\Delta m^2\cos2\theta$}
{wiki=Mikheyev–Smirnov–Wolfenstein_effect}
Matter changes the effective <mixing angle> and level splitting of propagating <neutrinos>. The electron-flavour <charged current> potential is $\sqrt2G_FN_e$ 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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