DIQUARK CONFINEMENT IN AN EXTENDED NJL MODEL

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 app roximation. The gap equation, obtained in a truncation scheme motivate d via a non-trivial quark-gluon vertex function, is solved to determin e the constituent quark mass if chiral symmetry is spontaneously broke n. In a second step, the Bethe-Salpeter equations for mesons and diqua rks beyond the ladder approximation are derived, taking care to preser ve Goldstone's theorem in the pion channel. While the obtained masses of pseudo-scalar and vector mesons are only moderately shifted compare d to the values in the ladder approximation, we observe that scalar di quarks disappear from the physical spectrum and therefore are confined . For axial-vector diquarks we observe indications that the same mecha nism may also work, but the NJL model allows no conclusive answer in t his channel. (C) 1997 Elsevier Science B.V.