Fermion Helicity Flip in Weak Gravitational Fields
  The helicity flip of a spin-${\textstyle \frac{1}{2}}$ Dirac particle interacting gravitationally with a scalar field is analyzed in the context of linearized quantum gravity. It is shown that massive fermions may have their helicity flipped by gravity, in opposition to massless fermions which preserve their helicity. 
R. Aldrovandi, G. E. A. Matsas, S. F. Novaes, and D. Spehler