Theory of the pi+ pi- atom
  The relativistic perturbative approach based on the Bethe-Salpeter (BS) equation is developed for the study of the characteristics of the hadronic pi+ pi- atom. The general expression for the atom lifetime is derived. Lowest-order corrections to the relativistic Deser-type formula for the atom lifetime are evaluated within the Chiral Perturbation Theory. The lifetime of the pi+ pi- atom in the two-loop order of the Chiral Perturbation Theory is predicted to be tau_1=(3.03\pm 0.10)\times 10^{-15}~s. 
M.A. Ivanov (BLThP, JINR, Dubna, Russia), V.E. Lyubovitskij (BLThP,   JINR, Dubna, Russia and Tomsk University, Russia), E.Z...