On the nature of the neutrino
  Assuming that one neutrino type with definite mass is described by a massive Dirac field operator, it is shown that the physical one-particle states for particles and antiparticles can be rotated to each other, irrespective of their helicity. This result is used to prove that the neutrino must necessarily be a Majorana particle. 
R. Romero
physics.gen-ph hep-th