  After a brief introduction on QCD and effective models in the first chapter,
I analyze the dependence of the QCD transition temperature on the quark (or
pion) mass in the second chapter. I found that a linear sigma model, which
links the transition to chiral symmetry restoration, predicts a much stronger
dependence of T_c on m_pi than seen in present lattice data for m_pi >~ 0.4
GeV. On the other hand, an effective Lagrangian for the Polyakov loop requires
only small explicit symmetry breaking to describe T_c(m_pi) in the above mass
range. In the third and fourth chapter, I study the linear sigma model with
O(N) symmetry at nonzero temperature in the framework of the
Cornwall-Jackiw-Tomboulis formalism. Extending the set of two-particle
irreducible diagrams by adding sunset diagrams to the usual Hartree-Fock (or
Hartree) contributions, I derive a new approximation scheme which extends the
standard Hartree-Fock (or Hartree) approximation by the inclusion of nonzero
decay widths.

High Energy Physics - Phenomenology