Independent particle motion and correlations in fermion systems

The independent-particle model explains many features of atomic nuclei and other fermion systems. The low-energy states of nearly closed-shell systems can be interpreted as having quasiparticles in single-particle orbitals. The difference between physical particles and quasiparticles results from the effects of correlations in the system. In this Colloquium the authors consider the consequences of these correlations. They discuss in particular, mainly for the case of nuclei, the quasihole strength z (spectroscopic factor) that gives the probability of the quasiparticle's being a physical particle. Results from both theory and experiment indicate that z similar to 0.65 and imply that only similar to 2/3 of the time a nucleon acts as an independent particle bound in an average potential. The fraction of similar to 1/3 of correlated nucleons is larger than believed in the past.