Color deconfinement and subhadronic matter: phase states and the role of constituent quarks

Major aspects of the subhadronic state of nuclear matter populated with deconfined color particles are reviewed. At high and even at rather low nuclear collision energies, this is expected to be a short-term quark–gluon plasma (QGP), but, seemingly, not only this. Emphasis is put on the self-consistency requirement that must be imposed on any phenomenological description of the evolution of a hot and dense nuclear medium as it expands (cools down) to the point where the final scattering of secondary particles starts. The view is argued and analyzed that massive constituent quarks should then play a major role at a certain cooling stage. A hypothesis is discussed regarding the existence of an intermediate stage (a valon plasma), allowing a consistent explanation of data on the mid-rapidity yields of various kinds of hadrons and direct dileptons ($\mathrm{e}^+\mathrm{e}^-$-pairs) in high-energy heavy-ion collisions.