Abstract

The pi(+)pi(-) atom decays - in the ground state - predominantly into two neutral pions. We present a general expression for the corresponding decay width in the framework of QCD (including photons). It contains all terms at leading and next-to-lending order in isospin breaking. The result allows one to evaluate the combination \a(0) - a(2)\ of pi pi S-wave scattering lengths from pi(+)pi(-) lifetime measurements, Like the one presently performed by the DIRAC experiment at CERN. (C) 1999 Published by Elsevier Science B.V. All lights reserved.