Collective effects in the radiation pressure force

We discuss the role of diffuse, Mie, and cooperative scattering on the radiation pressure force acting on the center of mass of a cloud of cold atoms. Even though a mean-field ansatz (the \u201ctimed Dicke state\u201d), previously derived from a cooperative scattering approach, has been shown to agree satisfactorily with experiments, diffuse scattering also describes very well most features of the radiation pressure force on large atomic clouds. We compare in detail an incoherent, random walk model for photons and a diffraction approach to the more complete description based on coherently coupled dipoles. We show that a cooperative scattering approach, although it provides a quite complete description of the scattering process, is not necessary to explain the previous experiments on the radiation pressure force