  We consider the problem of existence of asymptotic observables in local
relativistic theories of massive particles. Let $\ti p_1$ and $\ti p_2$ be two
energy-momentum vectors of a massive particle and let $\De$ be a small
neighbourhood of $\ti p_1+\ti p_2$. We construct asymptotic observables
(two-particle Araki-Haag detectors), sensitive to neutral particles of
energy-momenta in small neighbourhoods of $\ti p_1$ and $\ti p_2$. We show that
these asymptotic observables exist, as strong limits of their approximating
sequences, on all physical states from the spectral subspace of $\De$.
Moreover, the linear span of the ranges of all such asymptotic observables
coincides with the subspace of two-particle Haag-Ruelle scattering states with
total energy-momenta in $\De$. The result holds under very general conditions
which are satisfied, for example, in $\la \phi^4_2$. The proof of convergence
relies on a variant of the phase-space propagation estimate of Graf.
