Constructing the covariant three-point vertices systematically

An algorithm is developed for efficiently constructing the Lorentz covariant effective three-point vertices of the decay of a particle into two daughter particles in which all the masses and spins of the three particles can be arbitrary. The closely related one-to-one correspondence between the helicity formalism and the covariant formulation is exploited for counting the number of independent terms and identifying the basic covariant three-point vertices. Assembling the basic operators according to the developed algorithm enables us to construct all the covariant three-point vertices systematically.