  Color confinement is one of the central issues in QCD so that there are
various interpretations of this feature. In this paper we have adopted the
interpretation that colored particles are not subject to observation just
because colored states are unphysical in the sense of Eq. (2.16). It is shown
that there are two phases in QCD distinguished by different choices of the
gauge parameter. In one phase, called the "confinement phase", color
confinement is realized and gluons turn out to be massive. In the other phase,
called the "deconfinement phase", color confinement is not realized, but the
gluons remain massless.

High Energy Physics - Theory