On the possible types of equations for zero-mass particles
  There are a number of papers dedicated to the description of free particles and antiparticles with zero mass and spin 1/2. A great many equations with different C, P, T properties have been proposed and the impression could be formed that there are many nonequivalent theories for zero-mass particles. The purpose or this paper is to show that it is not the case and to describe all nonequivalent equations. 
Wilhelm I. Fushchych and Anatoly G. Nikitin (Institute of Mathematics,   Kiev, Ukraine)