Crossover from fermionic to bosonic quantum gases

Correlations between a composite boson and a fermion pair are considered in the context of the crossover theory of fermionic to bosonic superfluidity. It is shown that such correlations are the minimal ingredients needed in a many-body theory to generate the right boson-boson scattering length in the Bose-Einstein limit of the crossover. This is applied to the formation of molecules in the normal phase which can be compared with experimental data in the available alkali systems.