Solving the four-dimensional $NN-\pi NN$ equations for scalars below meson-production threshold

The four-dimensional $NN-\pi NN$ equations are adapted to the case of scalar particles with a $\phi^2\sigma$ interaction Lagrangian, and solved for energies below the $\sigma$-production threshold. This is achieved in the approximation where $\phi\sigma$ scattering is dominated by the $s$-channel $\phi$-pole term. The importance of the removal of double-counting is investigated, and a detailed comparison of the results of a covariant coupled-channels formulation and the Bethe-Salpeter equation in the ladder and ladder plus crossed-box approximations is presented. A brief discussion of the extension of the method to energies above the $\sigma$-production threshold is given.