On the thermal mass shift of nucleons

The mass shift $\Delta m_N$ of a nucleon in the finite temperature pion gas is calculated in the case of an arbitrary nucleon momentum. A general formula is used which relates the in-medium mass shift $\Delta m(E)$ of a particle to the real part of the forward scattering amplitude $Re f(E)$ of this particle on constituents of the medium and its applicability domain is formulated. The mass shift of the nucleon in thermal equilibrium with pion gas is also calculated.