Auger decay, Spin-exchange, and their connection to Bose-Einstein   condensation of excitons in Cu_2O
  In view of the recent experiments of O'Hara, et al. on excitons in Cu_2O, we examine the interconversion between the angular-momentum triplet-state excitons and the angular-momentum singlet-state excitons by a spin-exchange process which has been overlooked in the past. We estimate the rate of this particle-conserving mechanism and find a substantially higher value than the Auger process considered so far. Based on this idea, we give a possible explanation of the recent experimental observations, and make certain predictions, with the most important being that the singlet-state excitons in Cu_2O is a very serious candidate for exhibiting the phenomenon of Bose-Einstein condensation. 
G. M. Kavoulakis (NORDITA, Denmark), and A. Mysyrowicz (Ecole   Polytechnique, Palaiseau, France)