  We address the scattering problem in two-dimensional integrable models,
focusing on the sine-Gordon theory. We use the S-matrix bootstrap approach
based on analytical properties of the S-matrix to compute scattering amplitudes
of the sine-Gordon model. We obtain original integral representations for the
sine-Gordon scattering amplitudes involving bound states. Calculation of
minimal form factors in this model follows directly from integral
representations of scattering amplitudes. We also propose the extension of the
sine-Gordon bootstrap equations to the space of conformal correlators known as
celestial amplitudes.
