Mass of the sine-Gordon soliton in the Hartree approximation

We derive the quantum corrections to the mass of the one-soliton solution of the sine-Gordon system in the Hartree approximation. In the weak-coupling limit we reproduce the semiclassical correction to the soliton mass. This happens only after a nontrivial cancellation of contributions related to the deformation of the soliton due to quantum fluctuations. Numerical results are obtained up to the critical value of the coupling constant as given by Coleman. In approaching the critical point we find an increasing number of discrete modes which seem to build up a new continuum with a lower mass.