Goldstone Theorem and Diquark Confinement Beyond Rainbow-Ladder   Approximation
  The quark Dyson-Schwinger equation and meson Bethe-Salpeter equation are studied in a truncation scheme that extends the rainbow-ladder approximation such that, in the chiral limit, the isovector, pseudoscalar meson remains massless. Quark-quark (diquark) correlations, which are bound in rainbow-ladder approximation, are destabilised by repulsive contributions that only appear at higher order in the Bethe-Salpeter kernel. The net effect of higher order terms on the meson bound-state masses is small. 
A. Bender, C. D. Roberts and L. v. Smekal