QCD Vacuum as Dual Superconductor: Quark Confinement and Topology
Résumé
Quantum chromodynamics exhibits two important nonperturbative phenomena: color confinement and chiral symmetry breaking. These dynamical vacuum effects determine the crucial features of the hadronic phase, such as mass gap generation and the absence of colored asymptotic physical states. Unfortunately, despite their fundamental importance for our understanding of nature, neither of these phenomena can be derived analytically, starting from QCD Lagrangian. There are, however, phenomenological models that allow us to understand the nonperturbative features of the QCD vacuum and put them together into a coherent picture. In this chapter, we discuss one of the most successful phenomenological approaches to QCD based on a representation of the gluonic vacuum as a dual superconductor. First, we review Abelian gauge models pos- sessing monopole condensation, which supports the formation of confining string and leads to mass gap generation. Then, we discuss how one can incorporate a similar mechanism in QCD, including the identification of monopole-like gluon configurations in the context of the topology of the color gauge group. Also, we describe the numerical results of first-principle simulations demonstrating that the QCD vacuum exhibits monopole condensation, which supports its dual superconducting properties and leads to the formation of the confining chromoelectric string. Furthermore, the numerical simulations show, somewhat unexpectedly, that the monopole condensate produces the chiral condensate and thus generates the dynamical breaking of the chiral symmetry. Thus, the dual superconductor mechanism describes the essential features of the QCD vacuum related to its confining and chiral features.