Consistent Theories of Free Dirac Particle without Singular Predictions

Dirac’s theory is not the unique consistent with physical principles specific of a free spin-one-half particle. In fact, we derive classes theories an elementary particle from principle Poincaré’s invariance and covariance position. The Dirac just one these theories, characterized by singular predictions, namely, zitterbewegung. Yet, class here derived contains families without predictions. For time being, experimental verifiability alternative restricted to predictions free-particle for ideal experiments.