Effect of spacetime geometry on neutrino oscillations

Abstract

Propagation of fermions in spacetime requires a spin connection, which can be split into a universal gravitational part and a non-universal ``contorsion'' part. The latter is non-dynamical and can be eliminated from the theory, leaving an effective four-fermion interaction with unknown coupling constants. The generic form of the contorsion-fermion coupling, and thus the four-fermion interaction, breaks chiral symmetry. This interaction affects all fermions -- in particular neutrinos passing through matter will notice a contribution to their effective Hamiltonian, just like the MSW effect coming from weak interactions, but diagonal in the mass basis rather than the flavor basis. Then there is a possibility that this geometrical contribution is not negligible for neutrinos passing through normal matter, provided the coupling constants are not too small. We calculate the matter potential due to this interaction and thus write the conversion and survival probabilities including its effect. We plot conversion probabilities of $\nu_\mu$ to $\nu_e$ and $\nu_\tau$ for a baseline of 1300 km, as well as their dependence on the CP phase, with and without the geometrical interaction. We also plot the survival probability of reactor $\bar{\nu}_e$ for different baselines.