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. 1 Vacuum condensates in QCD. Phenomenological and theoretical evidence There is now ample phenomenological evidence for the existence of a nontrivial structure in the QCD vacuum, containing both quark[1] and gluon[2] condensates. A good description of many hadronic quantities is obtained in the framework of the QCD sum rules[3], using as input the vacuum expectation values 〈qq〉 and 〈 F μν a F a μν 〉 . Analytical approximations to the QCD ground state also provide theoretical evidence for the existence of the condensates and, in addition, supply