Renormalized atomic interaction and quadrupole excitations of cold Fermi gas near Feshbach resonance

We present a model space particle-hole Greens function calculation for the quadrupole excitations of cold Fermi gas near Feshbach resonance using a simple model where atoms are confined in a harmonic oscillator potential. Both the Tamm-Dancoff and random phase approximations are employed. By summing up exactly the ladder diagrams between a pair of interacting atoms to all orders, we obtain a renormalized atomic interaction which has well defined and identical limits as the scattering length tends to $pm infty$. The experimentally observed abrupt rise in the excitation spectrum and its associated large decay width are satisfactorily reproduced by our calculation.