Thermodynamical Bethe Ansatz analysis in an SU(2)xU(1) symmetric sigma-model

Four different types of free energies are computed by both thermodynamical Bethe Ansatz (TBA) techniques and by weak coupling perturbation theory in an integrable one-parameter deformation of the O(4) principal chiral sigma-model (with SU(2)xU(1) symmetry). The model exhibits both `fermionic' and `bosonic' type free energies and in all cases the perturbative and the TBA results are in perfect agreement, strongly supporting the correctness of the proposed S matrix. The mass gap is also computed in terms of the Lambda parameters of the modified minimal substraction scheme and a lattice regularized version of the model.