Chew, G.F.; Frautschi, S.C.; Principle of Equivalence for all Strongly Interacting Particles within the S-Matrix Framework
Phys. Rev. Lett. 7 (1961) 394;

Reprinted in
Reprinted in Regge theory of low-p_{T} hadronic interactions, editor L. Caneschi, p. 7-10..

Motivation
The notion, inherent in Lagrangian field theory, that certain particles are fundamental while others are complex, is becoming less and less palatable for baryons and mesons as the number of candidates for elementary status continues to increase. Sakata has proposed that only neutron, proton, and are elementary, but this choice is rather arbitrary, and strong-interaction consequences of the Sakata model merely reflect the established symmetries. Heisenberg
some years ago proposed an underlying spinor field that corresponds to no particular particle but which is supposed generate all the observed particles on an equivalent basis. The spirit of this approach satisfies Feynman's criterion that the correct theory should not allow a decision as to which particles are elementary, but it has proved difficult to find a convincing mathematical framework in which to fit the fundamental spinor field. On the other hand, the analytically continued S-matrix
with only those singularities required by unitarity has progressively, over past half decade, appeared more and more promising as a basis for describing the strongly interacting particles. Our purpose here is to propose a formulation of the Feynman principle within the S-matrix framework. (Extracted from the introductory part of the paper.).

Related references More (earlier) information appears in G. F. Chew and S. C. Frautschi, Phys. Rev. 123 (1961) 1478;
G. F. Chew and S. C. Frautschi, Phys. Rev. 124 (1961) 264;
See also G. F. Chew, The S-Matrix Theory of Strong Interactions W. A. Benjamin and Company, New York (1961);
V. N. Gribov, Nucl. Phys. 22 (1961) 249;
R. H. Dalitz and S. F. Tuan, Ann.Phys. 10 (1960) 307;
T. Regge, Nuovo Cim. 14 (1959) 951;
T. Regge, Nuovo Cim. 18 (1960) 947;
M. Froissart, Phys. Rev. 123 (1961) 1053;
W. Heisenberg, Rev. of Mod. Phys. 29 (1957) 269;
S. Sakata, Progr. of Theor. Phys. 16 (1953) 686;
L. Castillejo, R. H. Dalitz, and F. J. Dyson, Phys. Rev. 101 (1956) 453;

Record comments
Invention of equivalence of elementary hadrons and hadronic resonances on the basis of Regge trajectories. Invention of the Chew-Frautschi plot to classify hadrons. Invention of the vacuum pomeron trajectory.