Massive vector particles with spin zero

In this paper we show that a massive composite particle with spin zero can be a vector particle. If a massive particle is composed of a fermion and an antifermion with antiparallel spins and they are bound by a local interaction of the form ${\Psi }^{†}{\gamma }_0{\gamma }_{\mu }\Psi$, its field is a four-vector. We show that this composite particle with antiparallel spins cannot be transformed into a composite particle with parallel spins. If the pion or kaon is such a particle, this would explain how it can have spin 0 but still contain directional information as some experimental evidence indicates. (letter)