Quark Confinement and Deconfinement in QCD from the Viewpoint of Abelian-Projected Effective Gauge Theory

We give another derivation of quark confinement in QCD from the viewpoint of the low-energy effective Abelian gauge theory of QCD obtained via Abelian projection. It is based on the recently discovered Berezinskii-Kosterlitz-Thouless phase transition in the four-dimensional Abelian gauge theory. Moreover, we show that there exists a critical gauge coupling constant in QCD, above which confinement set in and below which there is no confinement. In a SU(N) gauge theory with $N_f$ flavored fermions, we argue that this leads to a critical value of fermion flavors $N_f^c$ for the confinement as well as the chiral symmetry breaking, which separates the deconfinement and chiral symmetric phase. A finite-temperature deconfinement transition is also discussed briefly.