Flavor-changing electromagnetic transitions

Abstract

We compute the $q_i{q}_j\gamma$ transition amplitude for off-shell and on-shell quarks with flavors $i\ne j$, in the standard weak-interaction model. The results we present are valid for all quark and photon momenta and for all external and internal quark masses. We discuss the Feynman rules and Ward-Takahashi identities in the 't Hooft—Feynman gauge. Our results can be applied to a variety of phenomena such as the electric dipole moment of the neutron, the process $e^{+}{e}^{-}\to t\overline{c}$, and flavor-changing transitions with either gluon or photon emission.