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Polaron formation in the vicinity of a narrow Feshbach resonance in atomic and exciton-polariton condensates

The polaronic system consisting of an impurity in a dilute Bose-Einstein condensate is considered in the presence of a narrow Feshbach resonance. For this purpose a coupled-channel model is used, which at the mean field level predicts the formation of quasiparticles that are a superposition of the impurity and the molecular states. The impurity-boson interactions and the coupling between the open and closed channels are then considered weak and a perturbative calculation of the corrections to the mean field results is presented. This allows to examine the properties of the quasiparticles, such as the lifetime and the effective mass. The model is applied to two physical systems: an impurity atom in a Bose-condensed atomic gas in 3D and a spin down lower polariton in a Bose-Einstein condensate of spin up lower polaritons in 2D. The model parameters are linked to the physical parameters by identifying the low energy T-matrix and applying a proper renormalization scheme.