Description
A low energy effective theory based on a microscopic multi-channel description of the atom-atom
interaction is derived for the scattering of alkali atoms in different hyperfine states [1]. This
theory describes all scattering properties, including medium effects, in terms of the singlet and
triplet scattering lengths and the range of the atom-atom potential and provides a link between a
microscopic description of Feshbach scattering and more phenomenological approaches based on
the treatment of the Feshbach molecule as a point boson. It permits the calculation of medium
effects on the resonance coming from the occupation of closed channel states. The examination
of such effects are demonstrated to be of particular relevance to an experimentally important
Feshbach resonance for 40 K atoms. We then discuss the case of a single impurity interacting
resonantly with a gas of fermions and show how it changes from being essentially a quasi-particle
excitation to a molecular excitation with increasing coupling strength [2].
