  We consider nonrelativistic superfluids where the global U(1)-symmetry is
spontaneously broken. At sufficiently long wavelengths, the relevant degree of
freedom is the massless Goldstone mode and we construct an effective low energy
theory for the Goldstone boson. The damping rate of collective excitations at
low energy is calculated. In the case of a weakly interacting Bose gas, we
recover the results by Beliaev, and by Hohenberg and Martin.
