Optical activity of neutrinos and antineutrinos

Using the one-loop helicity amplitudes for low-energy $\nu \gamma \to \nu \gamma$ and $\bar{\nu }\gamma \to \bar{\nu }\gamma$ scattering in the standard model with massless neutrinos, we study the optical activity of a sea of neutrinos and antineutrinos. In particular, we estimate the values of the index of refraction and rotary power of this medium in the absence of dispersion.