Helicity Precession of Spin-1/2 Particles in Weak Inertial and   Gravitational Fields
  We calculate the helicity and chirality effects experienced by a spin-1/2 particle subjected to classical electromagnetic and gravitational fields. The helicity evolution is then determined in the non-relativistic, relativistic, and ultra-relativistic regimes. We find that inertia-gravitation can distinguish between helicity and chirality. Helicity is not conserved, in general, even when the particles are massless. In this case, however, the inertial fields can hardly be applied to the fermions. 
Dinesh Singh, Nader Mobed, Giorgio Papini