No abstract available

[en] The DIRAC experiment aims to measure the lifetime of π+π- atoms in the ground state with 10% precision, using the 24 GeV/c proton beam of the CERN Proton Synchrotron. As the value of the above lifetime of order 10-15s is dictated by a strong interaction at low energy, the precise measurement of this quantity enables to determine a combination of S-wave pion scattering lengths to 5%. Pion scattering lengths have been calculated in the framework of chiral perturbation theory with high precision. Thus the accurate measurement of these values would submit the understanding of chiral symmetry breaking of QCD to a crucial test. Some preliminary results from the analysis of a 2000 data sample are presented

[en] The main objective of DIRAC experiment is the measurement of the lifetime τ of the exotic hadronic atom consisting of π⁺ and π^- mesons. The lifetime of this atom is determined by the decay mode π⁺ π^- → π⁰ π⁰ due to the strong interaction. Through the precise relationship between the lifetime and the S-wave pion-pion scattering length difference |a₀ - a₂| for isospin 0 and 2 (respectively), a measurement of τ with an accuracy of 10% will allow a determination of |a₀ - a₂| at a 5% precision level. Pion-pion scattering lengths have been calculated in the framework of chiral perturbation theory with an accuracy below 5%. In this way DIRAC experiment will provide a crucial test of the chiral symmetry breaking scheme in QCD effective theories at low energies