Selection Rules for Parafields and the Absence of Para Particles in Nature

Green's parafield quantization is reviewed. It is shown, both for a single field and for sets of fields, that all Fock-like representations of Green's trilinear commutation rules are realized by Green's ansatz with anticommuting (commuting) Bose (Fermi) component fields for para-Bose (para-Fermi) fields. Restrictions on the form of the interaction Hamiltonian density HI(x) are derived from the requirement that HI(x) be a paralocal operator. From these restrictions on HI selection rules on the S matrix are proved to all orders of perturbation theory. The most important such rule prohibits all reactions in which the total number of para particles of order $p>1$ in the initial and final states is one. This last selection rule, together with experimental information, leads to the conclusion that no presently known particle can be para.