The properties of the vacuum are addressed in two- and four-dimensional quark models for QCD. It is demonstrated that two-dimensional QCD (the ’t Hooft model) possesses only one possible vacuum state—the solution to the mass-gap equation, which provides spontaneous breaking of chiral symmetry (SBCS). On the contrary, the four-dimensional theory with confinement modeled by the linear potential with a Coulomb OGE interaction not only has a chirally noninvariant ground vacuum state, but possesses an excited vacuum replica, which also exhibits SBCS and can be realized as a metastable intermediate state of hadronic systems. We discuss the influence of this vacuum replica on physical observables as well as on the possibility of probing the vacuum background fields in QCD.

• Received 18 December 2001

DOI:https://doi.org/10.1103/PhysRevD.65.085026