Beliaev damping of quasi-particles in a Bose-Einstein condensate
  We report a measurement of the suppression of collisions of quasi-particles with ground state atoms within a Bose-Einstein condensate at low momentum. These collisions correspond to Beliaev damping of the excitations, in the previously unexplored regime of the continuous quasi-particle energy spectrum. We use a hydrodynamic simulation of the expansion dynamics, with the Beliaev damping cross-section, in order to confirm the assumptions of our analysis. 
N. Katz, J. Steinhauer, R. Ozeri, and N. Davidson