Gluon condensate and the vacuum structure of QCD
  Phenomenological evidence and analytic approximations to the QCD ground state suggest a complex gluon condensate structure. Exclusion of elementary fermion excitations by the generation of infinite mass corrections is a consequence. In addition the existence of vacuum condensates in unbroken non-abelian gauge theories, endows SU(3) and higher order groups with a non-trivial structure in the manifold of possible vacuum solutions, which is not present in SU(2). This may be related to the existence of particle generations. 
R. Vilela Mendes (Grupo de Fisica-Matematica, Complexo   Interdisciplinar, Univ. de Lisboa, Portugal)