OFF-SHELL DYNAMICS OF THE O(3) NLS MODEL BEYOND MONTE-CARLO AND PERTURBATION-THEORY

The off-shell dynamics of the O(3) non-linear sigma model is probed in terms of spectral densities and two-point functions by means of the f orm factor approach. The exact form factors of the spin field, Noether current, EM tensor and the topological charge density are computed up to six particles. The corresponding n less than or equal to 6 particl e spectral densities are used to compute the two-point functions, and are argued to deviate at most a few per mille from the exact answer in the entire energy range below 10(3) in units of the mass gap. To cove r yet higher energies we propose an extrapolation scheme to arbitrary particle numbers based on a novel scaling hypothesis for the spectral densities. It yields candidate results for the exact two-point functio ns at all energy scales and allows us to exactly determine the values of two, previously unknown, non-perturbative constants. (C) 1997 Elsev ier Science B.V.