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A classical theory of interacting relativistic constituent and composite particles is developed further. The Lorentz-invariant Lagrangian, a function of the single unmeasurable evolution parameter s, is considered for attractive and repulsive harmonic-oscillator forces acting pairwise between constituent particles. Nonrelativistic Newtonian equations of motion can be derived by letting c --t cc in "equal-time" solutions, but, in general, there is a "surplus" of solutions which have no nonrelativistic counterpart. These solutions are used to construct classical models of strongly interacting composite particles. Asymptotic selection rules and constituent confinement are postulated and lead to space-time conservation laws for systems of scattering composite particles. Constituent four-vectors are linear combinations of "kinematic" terms and "intrinsic" normal modes. The latter are identified with internal symmetries of the composite particles, which are labeled by sets of "intrinsic numbers" analogous to additive quantum numbers. Formation of two-and three-body composite particles follows an exact analogy to the color quark model, in which the meson is composed of a quark and an antiquark of the same color, and the baryon is formed from three quarks of three different colors. Scattering examples are given analogous to MM-MM, MB +MB, and BB +BB. The reactions take place through constituent exchange, and total intrinsic numbers are conserved. There are other similarities to quantum field theory, such as particle-antiparticle pair creation and annihilation, fixed relative values of internal angular momenta, fixed orbital angular momentum, and many-particle systems characterized by a vacuum state (lowest energy state) and the existence of virtual composite particles as well as physically observable composite particles.

With the aid of a covariant spin formalism, the kinematic constraint equations for helicity amplitudes are studied in a systematic way for all mass configurations, including the case of zero-mass particles. The complete set of constraints a t thresholds and pseudothresholds is given in a form convenient for calculation; that is, the coefficients of the helicity amplitudes are simple numerical ones (as opposed, for instance, to D functions). For nonzero-mass particles, the reduction of the constraints in terms of total spin amplitudes is shown to follow. Amplitudes with different values of total spin are not related a t thresholds or pseudothresholds.'A partial list of recent works would include Refs. 2-10, other works can be traced from them.