  We investigate matching conditions and threshold corrections between full and
effective theories based on the type I seesaw mechanism. In general, using an
intuitive Feynman diagrammatical approach, we compute the amplitudes before and
after integrating out heavy right-handed neutrinos at the matching scale. In
particular, we derive the one-loop matching conditions between the full and the
effective theories. The matching conditions of the parameters are influenced by
one-loop corrections to the corresponding vertices as well as wave function
corrections for the Higgs and the lepton fields. Our results are comparable to
earlier results based on a functional approach.
