Hydrodynamic modes in a trapped gas of metastable helium above the   Bose-Einstein transition
  This article describes experiments performed with an ultracold gas of metastable helium above the Bose-Einstein transition. The gas is trapped in an harmonic magnetic potential and collective excitations are induced. Both the frequency and the damping rate of the lowest monopole-quadrupole m = 0 mode of excitation are studied at different temperatures and compared to theoretical predictions. Results indicate that the gas goes from a collisionless regime towards a hydrodynamic regime as the elastic collision rate increases, when one goes down along the evaporative cooling ramp. However we find a discrepancy for the collisional parameters in comparison with predictions relying on the value of the scattering length previously estimated. Possible explanations are put forward in the final discussion. 
M. Leduc, J. Leonard, F. Pereira dos Santos, E. Jahier, S. Schwartz   and C. Cohen-Tannoudji