  The decay of a quasiparticle in a confined geometry, resulting from
electron-electron interactions, has been mapped onto the single-electron
problem of diffusion on a Cayley tree by Altshuler et al. [Phys.Rev.Lett. 78,
2803 (1997)]. We study an alternative model, that captures the strong
correlations between the self-energies of different excitations with the same
number of quasiparticles. The model has a recursion relation for the
single-particle density of states that is markedly different from the Cayley
tree. It remains tractable enough that sufficiently large systems can be
studied to observe the localization transition in Fock space predicted by
Altshuler et al.
