Pair correlations of scattered atoms from two colliding Bose-Einstein   Condensates: Perturbative Approach
  We apply an analytical model for anisotropic, colliding Bose-Einstein condensates in a spontaneous four wave mixing geometry to evaluate the second order correlation function of the field of scattered atoms. Our approach uses quantized scattering modes and the equivalent of a classical, undepleted pump approximation. Results to lowest order in perturbation theory are compared with a recent experiment and with other theoretical approaches. 
J. Chwedenczuk, P. Zin, M. Trippenbach, A. Perrin, V. Leung, D. Boiron   and C. I. Westbrook