2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/52016The inverse time ordered Green's function is defined in a covariant three-dimensional formalism using the diagram approach.The equation of motion of a two-quark composite system in an external electromagnetic field up to the second order in the field strength is obtained. The polarizability of the $π$-meson is calculated using this formalism. The mean square radius of the two-quark system is calculated from the wave functions of the system. In addition, radiative corrections are calculated for both the polarizability and the mean square radius.5 pagesHigh Energy Physics - Theorytext