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The correlated motion of a positron surrounded by electrons is a fundamentalmany-body problem. We approach this by modeling the momentum density ofannihilating electron-positron pairs using the framework of reduced densitymatrices, natural orbitals and natural geminals (electron-positron pair wavefunctions) of the quantum theory of many-particle systems. We find that anexpression based on the natural geminals provides an exact, unique and compactexpression for the momentum density. The natural geminals can be used to defineand determine enhancement factors for enhancement models going beyond theindependent-particle model for a better understanding of results of positronannihilation experiments.

Enhancement models of momentum densities of annihilating electron-positron pairs: the many-body picture of natural geminals