The integrable Bullough-Dodd model under celestial holography

We study celestial amplitudes for the S-matrix of the 2d integrable Bullough-Dodd model. This model has bound states that appear as poles in the physics strip of its 2d S-matrix, which complicates the computation of celestial amplitudes. However, it turns out that the celestial amplitudes are, in fact, well-structured. The celestial bootstrap (arising from the unitarity and crossing symmetry of 2d S-matrix) can be decomposed into a finite-dimensional linear space, whose base-integrals evaluate into harmonic numbers. This clean structure replaces the complicated integration with simple algebra of elementary functions, and the celestial bootstrap reduces to a programmable recursion process of simple algebra. Interestingly, this linear space has a subspace that happens to cover the celestial bootstrap of the Sinh-Gordon model studied by 2209.02776. So the celestial dual of these 2d integrable models turns out to be 'bootstrapable' in the practical sense, that is, a programmable recursion process.