Can someone please explain what is meant by "helicity flip suppression" and how this mechanism operates? (I'd like to see an explicit amplitude and/or cross section if possible.) I've been reading papers in which a Majorana fermion self-annihilates into some resonance (say, a Z). Fermi-Dirac statistics at the annihilation vertex require these incoming fermions to have opposite spins, and conservation of angular momentum requires the final state fermion and antifermion (coming from the Z decay) to have opposite spins as well. Somehow this restriction yields a suppression factor equal to the final state fermion mass. I'd like to see exactly how this effect plays out.