Are occupation numbers observable?

The question of whether occupation numbers and momentum distributions of nucleons in nuclei are observables is considered from an effective field theory perspective. Field redefinitions lead to variations that imply the answer is negative, as illustrated in the interacting Fermi gas at low density. Implications for the interpretation of (e, e'p) experiments with nuclei are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.