Multipair contributions to the spin response of nuclear matter

We analyse the effect of non-central forces on the magnetic susceptibility of degenerate Fermi systems. These include the presence of contributions from transitions to states containing more than one quasiparticle-quasihole pair, which cannot be calculated within the framework of Landau Fermi-liquid theory, and renormalization of the quasiparticle magnetic moment, as well as explicit non-central contributions to the quasiparticle interaction. Consequently, the relationship between the Landau parameters and the magnetic susceptibility for Fermi systems with non-central forces is considerably more complicated than for systems with central forces. We use sum-rule arguments to place a lower bound on the contribution to the static susceptibility coming from transitions to multipair states.