We derive the exact contribution of two spinons to the dynamical correlation function of the spin-1/2 Heisenberg model. For this, we use the isotropic limits of the exact form factors that have been recently computed through the quantum affine symmetry of the anisotropic Heisenberg model XXZ.

The nucleon mass shift is calculated using chiral counting arguments and a virial expansion, without and with the ⌬. At all temperatures, the mass shift and damping rate are dominated by the ⌬. 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 color͒ limit. We show that unitarity implies that the concepts of thermal shifts are process dependent. ͓S0556-2821͑96͒03317-6͔

The partition function of two-dimensional solitons in a heat bath of mesons is worked out to one-loop. For temperatures large compared to the meson mass, the free energy is dominated by the meson-soliton bound states and the zero modes, a consequence of Levinson's theorem. Using the Bethe-Uhlenbeck formula we compare the soliton energyshift to the shift expected in the pole mass using a virial expansion. We construct the partition function associated to a fast moving soliton at finite temperature, and found that the soliton thermal inertial mass is no longer constrained by Poincare's symmetry.At finite temperature, the concept of quasiparticles is process dependent.

The exact form factors of the Heisenberg models XXX and XXZ have been recently computed through the quantum affine symmetry of XXZ model in the thermodynamic limit. We use them to derive an exact formula for the contribution of two spinons to the dynamical correlation function of XXX model at zero temperature.