Abstract

We construct an effective non-relativistic quantum field theory that describes bound states of pi (+)pi (-) pairs and their hadronic decays. We then derive a general expression for the lifetime of the ground state at next-to-leading order in isospin breaking. Chiral perturbation theory allows one to relate the decay rate to the two S-wave pi pi scattering lengths and to several low-energy constants that occur in the chiral Lagrangian. Recent predictions for the scattering lengths give tau= (2.9 +/-0.1) x 10(-15) s. This result may be confronted with pi (+)pi (-) lifetime measurements, like the one presently carried out at CERN.