  In a Nambu-Jona-Lasinio model supplemented with an infrared cutoff in
addition to the ultraviolet cutoff we study the issue whether diquarks are
confined when the model is extended beyond the rainbow-ladder approximation.
The gap equation, obtained in a truncation scheme motivated via a nontrivial
quark-gluon vertex function, is solved to determine the constituent quark mass
if chiral symmetry is spontaneously broken. In a second step, the
Bethe-Salpeter equations for mesons and diquarks beyond ladder approximation
are derived, taking care to preserve Goldstone's theorem in the pion channel.
While the obtained masses of pseudoscalar and vector mesons are only moderately
shifted compared to the values in ladder approximation, we observe that scalar
diquarks disappear from the physical spectrum and therefore are confined. For
axialvector diquarks we observe indications, that the same mechanism may also
work, but the NJL model allows no conclusive answer in this channel.
