Determination of ππ scattering lengths from measurement of π+π− atom lifetime

The DIRAC experiment at CERN has achieved a sizeable production of π+π− atoms and has significantly improved the precision on its lifetime determination. From a sample of 21 227 atomic pairs, a 4% measurement of the S-wave ππ scattering length difference |a0−a2|=(0.2533−0.0078+0.0080|stat|syst−0.0073+0.0078)Mπ+−1 has been attained, providing an important test of Chiral Perturbation Theory.