pi+pi- Atom in Chiral Theories
  We review the existing theoretical approaches to the study of the observables of hadronic atoms. In the relativistic perturbative approach based on the Bethe-Salpeter equation and Chiral Perturbation Theory we derive the general expression for the pi+pi- atom lifetime. The lowest-order correction to the relativistic Deser-type formula for the atom width is predicted to be 6.1\pm 3.1 %. 
M. A. Ivanov (JINR, Dubna), V. E. Lyubovitskij (JINR, Dubna and Tomsk   State University), E. Z. Lipartia (JINR, Dubna and I...