  The non-perturbative BRST quartet mechanism in infrared Landau gauge QCD is
presented. It is demonstrated that positivity violation for transverse gluons
allows to identify the gluon's non-perturbative BRST quartet. To describe the
respective BRST-daughter state a truncated Bethe-Salpeter equation for the
gluon-ghost bound state is investigated. An analogous construction for quarks
yields a truncated Bethe-Salpeter equation for the quark-ghost bound state. The
gluon-ghost bound state equation in two space-time dimensions has been
numerically solved.
