Optical activity in an isotropic gas of electrons with a preferred helicity.

An isotropic gas of electrons with a preferred spin helicity is shown to be optically active. Simultaneous eigenfunctions of the Dirac Hamiltonian and the helicity operator are constructed and used to derive explicit expressions for vertex functions for helicity states. The (covariant) response tensor is calculated for an electron gas described in terms of a spin-dependent occupation number. The possibility of detecting optical activity in an electron gas is discussed briefly.