Tachyons or Antiparticles?
  This article proves that elementary particles can violate the second postulate of the Special Theory of Relativity. According to Heisenberg's principle, superluminal particles are allowed in the theoretical framework of Quantum Mechanics. In this paper the tachyons, are identified with the unitary irreducible representations of the "full" Poincare group (Poincare group plus space-time reflections.) on the past time-like cone. 
Jose N. Pecina-Cruz