The proposed experiment aims to measure the lifetime of 
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 
s 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
and values predicted at the same level of accuracy have, up to now, never
been confronted with accurate experimental data. Such a measurement would
submit the understanding of chiral symmetry breaking of QCD to a crucial
test.