Evgenii L. Feinberg

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 (e+e−-pairs) in high-energy heavy-ion collisions.

“Free” constituent quarks and dilepton production in heavy-ion collisions

An approach is suggested that invokes vitally the notion of constituent massive quarks (valons) that can survive and propagate rather than hadrons (except of pions) within hot and dense matter formed below the chiral-transition temperature in the course of heavy-ion collisions at high energies. This approach is shown to be quite good for describing the experimentally observed excess in the dilepton yield at masses 250≤Mee≤700 MeV over the prompt-resonance-decay mechanism (CERES cocktail) predictions. In certain aspects, it appears to be even more successful than conventional approaches: it seems to match the data somewhat better at dilepton masses below the two-pion threshold and below the ρ-meson peak, as well as at higher dilepton masses (beyond the φ-meson one). The approach implies no specific assumptions on the special features of phase transitions in expanding nuclear matter, and the ideal gas approximation is motivated to be still workable for describing the pion-valonic system under consideration.