The phase transition restoring chiral symmetry at finite temperatures is considered in a linear $\sigma$ model. For three or more massless flavors, the perturbative $\epsilon$ expansion predicts the phase transition is of first order. At high temperatures, the ${\mathrm{U}}_A\mathrm{}\left(1\right)\mathrm{}$ symmetry will also be effectively restored.

• Received 27 October 1983

DOI:https://doi.org/10.1103/PhysRevD.29.338