  Neutrino mixing is caused by the fact that neutrino flavors are not
eigenstates of the free Hamiltonian. This causes oscillations among different
neutrino flavors. When neutrinos pass through a medium, weak interactions
produce different effective masses for neutrinos of different flavors, leading
to a modification of the mixing parameters. In curved spacetime there is an
additional contribution to neutrino Hamiltonian from a torsion-induced
four-fermion interaction, which also causes neutrino mixing while propagating
through fermionic matter. We provide an outline of the calculation of this
effect on neutrino oscillation.
