  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.
