Lifetime measurement of atoms consisting of pi+ and pi- mesons

The lifetime measurement of pi+pi- atoms (A2pi) with 10% precision provides in a model independent way the difference between the S-wave pion-pion scattering lengths for isospin 0 and 2, |a0 -a2|, with 5% accuracy. The scattering lengths a0 and a2 have been calculated in Chiral Perturbation Theory (ChPT) with a precision better than 2.5%. Therefore, such a measurement will be a sensitive check of the understanding of chiral symmetry breaking in QCD by giving an indication about the value of the quark condensate, an order parameter of QCD. The method of observation and lifetime measurement of A2pi are discussed. Data on A2pi production in Pt, Ni and Ti targets and preliminary results for the lifetime measurement are presented.