  In order to avoid the difficulties encountered by relativistic quantum theory
of single particles, we pursue a deductive development of the theory from
physical principles, without canonical quantization, by making use of
group-theoretical methods. Our work has pointed out the necessity of new
classes of irreducible representations of the Poincar\'e group the quantum
theory of a particle can be based on. For spin 0 particle, four inequivalent
theories are completely determined, with fundamental differences with respect
to Klein-Gordon theory.
