Chiral symmetry restoration in linear sigma models with different numbers of quark flavors

Chiral symmetry restoration at nonzero temperature is studied in the framework of the $O(4)$ linear sigma model and the U(Nf)ᵣ×U(Nf)ₗ linear sigma model with Nf=2,3 and 4 quark flavors. We investigate the temperature dependence of the masses of the scalar and pseudoscalar mesons, and the nonstrange, strange, and charm condensates within the Hartree approximation as derived from the Cornwall-Jackiw-Tomboulis formalism. We find that the masses of the nonstrange and strange mesons at nonzero temperature depend sensitively on the particular symmetry of the model and the number of light quark flavors Nf. On the other hand, because of the large charm quark mass, neither do charmed mesons significantly affect the properties of the other mesons, nor do their masses change appreciably in the temperature range around the chiral symmetry restoration temperature. In the chiral limit, the transition temperatures for chiral symmetry restoration are surprisingly close to those found in lattice QCD.