Thermodynamical Bethe Ansatz analysis in an SU(2)xU(1) symmetric σ-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 σ-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 Λ parameters of the modified minimal subtraction scheme and a lattice regularized version of the model