[Li, Bao-Chun] Shanxi Univ, Dept Phys, Taiyuan 030006, Shanxi, Peoples R China
; [Li, Bao-Chun] Chinese Acad Sci, Inst High Energy Phys, Beijing 100049, Peoples R China
; [Wang, Li-Li] Shanxi Univ, Dept Math, Coll Business, Taiyuan 030031, Shanxi, Peoples R China
We investigate the entropy in a scalar O(N) model with a spontaneously broken symmetry. The Cornwall-Jackiw-Tomboulis formalism is applied to derive the entropy in the Hartree and Hartree-Fock approximations. In contrast to the Hartree approximation where the particles are quasiparticles with vanishing decay width, the Hartree-Fock approximation contains exchange contributions, which lead to nonzero decay width. We find that the sunset diagrams contribution is very small. Furthermore, the decay widths from these diagrams are discussed in the weak and strong coupling cases.