As it is well known, one can lower the energy of the trivial perturbation QCD vacuum by introducing a nonvanishing chromomagnetic field strength. This happens because radiative corrections produce an effective action of the form [Formula: see text] with f′(y0) = 0 for some y0≠0. However, a vacuum with a nonzero field strength is not consistent with Poincaré Invariance (PI). Generalizing this type of effective action by introducing, in the simplest way, a four-index field strength ∂[μAναβ], which can have an expectation value without violating PI, we are lead to an effective action that can describe both a confinement phase and a perturbative phase of the theory. In the unconfined phase, the four-index field strength does not introduce new degrees of freedom, while in the confined phase both four-index field strength and ordinary gauge fields are not true degrees of freedom. The matching of these phases through membranes that couple minimally to the three-index potentials from which the four-index field strength derive, leads automatically to the MIT bag boundary conditions for the gauge fields living inside the bubble containing the perturbative phase.
Probing the QCD vacuum with an Abelian chromomagnetic field: A study within an effective model
