QCD PHASE-TRANSITIONS IN AN EFFECTIVE FIELD-THEORY

We analyze the transitions between hadronic and quark-gluon phases of QCD using an effective low-energy lagrangian for chiral and scale invariance, which embodies quark and gluon condensation. We argue that the quark condensation and deconfinement transitions take place at the same temperature, Tq, and that gluon condensation and confinement occur at a common temperature Tg. The requirement that scale invariance be spontaneously broken before quarks condense imposes Tq ≤ Tq. The gluon transition is in general first order, whereas the quark transition may be second order if Tq < Tg, or first order if Tq = Tg, which is favoured for large Nc. We argue on general grounds and demonstrate with several model calculations that massive hadrons play a key role in driving the gluon transition, and suggest that Tg is below the Hagedorn temperature. We also estimate the surface tension for hadronic bubbles forming in a quark-gluon plasma, finding it to be somewhat too small for inhomogeneities to affect the standard calculations of cosmological nucleosynthesis. © 1990.