Investigations of the perturbativeness of the particle-hole channel in nuclear matter

Conventional nuclear many-body calculations are strongly non-perturbative in the inter-nucleon interactions. This non-perturbative behavior arises from several sources: 1. a strongly repulsive short-range interaction, 2. a tensor force, e.g. from pion exchange, which is highly singular at short-distances, 3. the presence of low-energy bound states or nearly bound states (in the S waves). The Renormalization Group based effective potentials exploit the insensitivity of the long-distance physics to the details of the short-distance physics. When these effective potentials are used in nuclear many-body calculations, it has been observed that these sources of non-perturbative behavior are density and/or resolution scale dependent. As a result, the bulk properties of nuclear matter turn out to be perturbative at least in the particle-particle channel. In this talk the question of ``perturbativeness'' of the particle-hole channel in nuclear matter is investigated by comparing the exact RPA correlation energy to a perturbative one. The cut-off and/or regulator dependence of these result give us an estimate of size of the missing many-body forces.