Two-point correlation function in integrable QFT with anti-crossing symmetry

The two-point correlation function of the stress-energy tensor for the $Phi_{1,3}$ massive deformation of the non-unitary model ${cal M}_{3,5}$ is computed. We compare the ultraviolet CFT perturbative expansion of this correlation function with its spectral representation given by a summation over matrix elements of the intermediate asymptotic massive particles. The fast rate of convergence of both approaches provides an explicit example of an accurate interpolation between the infrared and ultraviolet behaviours of a Quantum Field Theory.