Feshbach resonances in ultracold gases
  In this chapter, we describe scattering resonance phenomena in general, and focus on the mechanism of Feshbach resonances, for which a multi-channel treatment is required. We derive the dependence of the scattering phase shift on magnetic field and collision energy. From this, the scattering length and effective range coefficient can be extracted, expressions which are particularly useful for ultracold gases. 
S. J. J. M. F. Kokkelmans
cond-mat.quant-gas