  We develop Haag-Ruelle scattering theory for Wigner particles in local
relativistic Quantum Field Theory without assuming mass gaps or any other
restrictions on the spectrum of the mass operator near the particle masses. Our
approach is based on the Reeh-Schlieder property of the vacuum state. It is
shown that a strengthened variant of this property, concerning the relative
approximation error for single-particle states, implies the existence of
scattering states.
