Global properties of nucleus-nucleus collisions

Abstract

QCD as a theory of extended, strongly interacting matter is familiar from big bang evolution which, within the time interval from electro-weak decoupling (10-12 s) to hadron formation (5 × 10-6 s), is dominated by the expansion of quark-gluon matter, a color conducting plasma that is deconfined. In the 1970s deconfinement was predicted [1-5] to arise from the newly discovered "asymptotic freedom" property of QCD; i.e., the plasma was expected to be a solution of perturbative QCD at asymptotically high square momentum transfer Q2, or temperature T Thus the quark-gluon plasma (QGP) was seen as a dilute gas of weakly coupled partons. This picture may well hold true at temperatures in the GeV to TeV range. However, it was also known since R. Hagedorns work [6] that hadronic matter features a phase boundary at a very much lower temperature, T (H) = 170MeV. As it was tempting to identify this temperature with that of the cosmological hadronization transition, thus suggesting T (H) = T (crit), the QGP state must extend downward to such a low temperature, with Q2 ≤ 1GeV2, and far into the non-perturbative sector of QCD, and very far from asymptotic freedom. The fact that, therefore, the confinement-deconfinement physics of QCD, occurring at the partonhadron phase boundary, had to be explained in terms other than a dilute perturbative parton gas was largely ignored until rather recently, when laboratory experiments concerning the QGP had reached maturity. © 2010 Springer-Verlag Berlin Heidelberg.