The nucleon mass shift is calculated using chiral counting arguments and a virial expansion, without and with the $\Delta$. At all temperatures, the mass shift and damping rate are dominated by the $\Delta$. Our results are compared with the empirical analysis of Leutwyler and Smilga, as well as results from heavy baryon chiral perturbation theory in the large $N_c$ (number of color) limit. We show that unitarity implies that the concepts of thermal shifts are process dependent.

• Received 18 March 1996

DOI:https://doi.org/10.1103/PhysRevD.54.5536