(Pi+Pi-) Atom in Chiral Perturbation Theory
  Hadronic (Pi+Pi-) atom is studied in the relativistic perturbative approach based on the Bethe-Salpeter equation. 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. 
M. A. Ivanov (JINR, Dubna), V. E. Lyubovitskij (JINR, Dubna and Tomsk   State University), E. Z. Lipartia (JINR, Dubna and I...