  We formulate a generalized concept of asymptotic completeness and show that
it holds in any Haag-Kastler quantum field theory with an upper and lower mass
gap. It remains valid in the presence of pairs of oppositely charged particles
in the vacuum sector, which invalidate the conventional property of asymptotic
completeness. Our result can be restated as a criterion characterizing a class
of theories with complete particle interpretation in the conventional sense.
This criterion is formulated in terms of certain asymptotic observables
(Araki-Haag detectors) whose existence, as strong limits of their approximating
sequences, is our main technical result. It is proven with the help of a novel
propagation estimate, which is also relevant to scattering theory of quantum
mechanical dispersive systems.
