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, T_q, and that gluon condensation and confinement occur at a common temperature T_g. The requirement that scale invariance be spontaneously broken before quarks condense imposes T_q ≤ T_q. The gluon transition is in general first order, whereas the quark transition may be second order if T_q < T_g, or first order if T_q = T_g, which is favoured for large N_c. 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 T_g 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.