Exact form factors in integrable quantum field theories: the sine-Gordon model

We provide detailed arguments on how to derive properties of generalized fo rm factors, originally proposed by one of the authors (M.K.) and Weisz twen ty years ago, solely based on the assumption of "maximal analyticity" and t he validity of the LSZ reduction formalism. These properties constitute con sistency equations which allow the explicit evaluation of the n-particle fo rm factors once the scattering matrix is known. The equations give rise to a matrix Riemann-Hilbert problem. Exploiting the "off-shell" Bethe ansatz w e propose a general formula for form factors for an odd number of particles . For the sine-Gordon model alias the massive Thirring model we exemplify t he general solution for several operators. In particular we calculate the t hree-particle form factor of the soliton field, carry out a consistency che ck against the Thirring model perturbation theory and thus confirm the gene ral formalism. (C) 1999 Elsevier Science B.V.