  We analyze the structure of higher-order radiative corrections for processes
with unstable particles. By subsequently integrating out the various scales
that are induced by the presence of unstable particles we obtain a hierarchy of
effective field theories. In the effective field theory framework the
separation of physically different effects is achieved naturally. In
particular, we automatically obtain a separation of factorizable and
non-factorizable corrections to all orders in perturbation theory. At one loop
this treatment is equivalent to the double-pole approximation (DPA) but
generalizes to higher orders and, at least in principle, to beyond the DPA. It
is known that one-loop non-factorizable corrections to invariant mass
distributions are suppressed at high energy. We study the mechanism of this
suppression and obtain estimates of higher-order non-factorizable corrections
at high energy.
