  We study the O(N) linear sigma model with spontaneous symmetry breaking at
finite temperature in the framework of the two-particle point-irreducible
(2PPI) effective action. We go beyond the Hartree approximation by including
the two-loop contribution, i.e., the sunset diagram. A phase transition of
second order is found, whereas it is of first order in the one-loop Hartree
approximation. Furthermore, we show the temperature-dependence of the
variational mass parameters and comment on their relation to the physical sigma
and pion masses.
