  The off-shell dynamics of the O(3) nonlinear sigma-model is probed in terms
of spectral densities and two-point functions by means of the form factor
approach. The exact form factors of the Spin field, Noether-current, EM-tensor
and the topological charge density are computed up to 6-particles. The
corresponding $n\leq 6$ particle 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
cover 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 functions at all
energy scales and allows us to exactly determine the values of two, previously
unknown, non-perturbative constants.

High Energy Physics - Theory;High Energy Physics - Lattice