Medium modification of two-particle scattering in nonideal Bose systems
  Medium modification of scattering properties in interacting Bose systems are considered by solving the Bethe-Salpeter equation. An equation of state for the normal phase (generalized Beth-Uhlenbeck formula) is given using the in-medium phase shifts to include two-particle correlations. Conclusions are drawn for systems of bosonic atoms with repulsive interaction such as sodium $^{23}$Na and rubidium $^{87}$Rb. It is shown that the in-medium scattering length and the absolute value of the in-medium scattering phase shift for low scattering energies increase with density. 
H. Stein, K. Morawetz, and G. R\"opke