Order of the chiral phase transtion in the sigma models
  The chiral phase transition at a certain critical temperature is a restoration mechanism of the chiral symmetry, broken by the nonzero mass of quarks and mesons. The transition can be studied through several models, among which are the sigma models. An analysis is made on the linear and nonlinear sigma models with different approximations, and we show that the transition is of second order in both cases. 
F. A. Correa and J. Morales