Chiral phase transition and instanton–anti-instanton molecules

In this paper we explore the idea that the chiral phase transition in QCD can be described as a transition from a disordered instanton liquid to a strongly correlated phase of polarized instanton--anti-instanton molecules. We calculate the degree of polarization of the molecules as a function of the temperature and show that the resulting T dependence of the fermion determinant drives the chiral phase transition. We also show how the polarization of the molecules can lead to a nontrivial behavior of the energy density and pressure. Finally, we study the effect of the presence of molecules on the propagation of quarks at T{~}Tc. We derive the corresponding effective interaction and find that the strength in the scalar-pseudoscalar channel is four times the strength in the vector--axial-vector channel which agrees with recent lattice QCD simulations. We give results for the quark condensates as well as mesonic and baryonic correlation functions and find that the ``screening masses'' of chiral partners become equal for T>Tc, where we still observe substantial attraction in the scalar-pseudoscalar meson channels.