On the lifetime of the π+π− atom

The π+π− 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-leading order in isospin breaking. The result allows one to evaluate the combination |a0−a2| of ππ S-wave scattering lengths from π+π− lifetime measurements, like the one presently performed by the DIRAC experiment at CERN.