  Recent large-volume lattice simulations have established that, in the Landau
gauge, the gluon propagator is infrared-finite. The most natural way to explain
this observed finiteness is the generation of a nonperturbative,
momentum-dependent gluon mass. Such a mass may be generated gauge-invariantly
by employing the Schwinger mechanism, whose main assumption is the dynamical
formation of massless bound-state excitations. In this work we demonstrate that
this key assumption is indeed realized by the QCD dynamics. Specifically, the
Bethe-Salpeter equation describing the aforementioned massless excitations is
derived and solved under certain approximations, and non-trivial solutions are
obtained.
