Helicity Zero Particles
  In this paper we consider the possibility that a vector particle with mass might exist in only one helicity state, rather than the usual three states with helicity equal to +1, -1, and 0. Massless particles, of course, need only have one helicity state. (For invariance under parity, they need two.) We show that a massive vector particle can exist only in the helicity-0 state, if it is composed of a fermion-antifermion pair and they are massless. This requires the mass to be generated by the interaction between the massless particles. An interaction of the form Psi^dagger i gamma_4 gamma_mu Psi is attractive between particle and antiparticle and preserves helicity. Methods for experimentally distinguishing an helicity-0 vector particle from both a spin-0 pseudoscalar particle and a spin-1 vector particle are discussed. 
W. A. Perkins