Getting the elastic scattering length by observing inelastic collisions   in ultracold metastable helium atoms
  We report an experiment measuring simultaneously the temperatureand the flux of ions produced by a cloud of triplet metastablehelium atoms at the Bose-Einstein critical temperature. The onsetof condensation is revealed by a sharp increase of the ion fluxduring evaporative cooling. Combining our measurements withprevious measurements of ionization in a pure BEC,we extract an improved value of the scattering length$a=11.3^{+2}_{-1}$ nm. The analysis includes corrections takinginto accountthe effect of atomic interactions on the criticaltemperature, and thus an independent measurement of the scatteringlength would allow a new test of these calculations. 
Signe Seidelin (LCFIO), Rodolphe Hoppeler (LCFIO), Olivier Sirjean   (LCFIO), Denis Boiron (LCFIO), Alain Aspect (LCFIO), Ch...