On a pointlike relativistic massive and spinning particle

A pointlike massive and spinning relativistic particle is described as a confined system of two massless directly interacting spinning constituents. The approach is Hamiltonian. The employed phase space is, thus, a symplectic vector space equipped with global canonical and Poincaré-covariant twistor coordinates. The Poincaré-invariant generator of the phase space motion does not represent the energy of the total system. Consequently, the evolution parameter cannot be identified with the time. The generating function, however, makes the position four vector and the proper time of the composite massive and spinning system into dynamical variables, i.e., functions of the evolution parameter. The phase flow may thus be interpreted as a simple particle dynamics in Minkowski space. In analogy with the definition of Bakamjian and Thomas for the center of energy of a relativistic massive and spinning particle, a definition of the center of energy of a massless particle with nonvanishing helicity is presented.