Description of a Particle with Arbitrary Mass and Spin

A Lorentz covariant description of a particle and antiparticle with spin s=0, 1/2, 1, ⋯ and finite rest mass is given in this paper. The wave function has $2(2s+1)$ components so that no auxiliary conditions are needed. The basic idea is to postulate the rest-system Hamiltonian and make the Lorentz transformation to the laboratory system. An algorithm which is a generalization of the Foldy-Wouthuysen transformation is found for constructing the laboratory-system Hamiltonian and polarization operators.