Quark self-energy beyond the mean field at finite temperature

The Nambu–Jona-Lasinio model, an effective low-energy model of QCD, is extended to the next to the leading order in the 1/${\mathit{N}}_{\mathit{c}}$ expansion at finite temperature and density. The contributions to the quark self-energy and the constituent quark mass from the meson dressing are considered in a perturbative approach about the mean field. In particular, the temperature dependence of the quark mass is shown numerically at zero chemical potential. The correction to the quark mass from the meson dressing amounts to 20% compared to the result of the leading order at low temperature, and rapidly approaches zero at high temperature.