Energy-dependent damping of excitations over an elongated Bose-Einstein condensate

We measure the damping of excitations in cigar-shaped Bose-Einstein condensates of atomic vapor. By using postselection, excitation line shapes of the total population are compared with those of the undamped excitations. We find that the damping depends on the initial excitation energy of the decaying quasiparticle, as well as on the excitation momentum. We model the condensate as an infinite cylinder and calculate the damping rates of the different radial modes. The derived damping rates are in good agreement with the experimentally measured ones. The damping rates strongly depend on the destructive interference between pathways for damping, due to the quantum many-body nature of both excitation and the vacuum modes.