Determination of $\pi\pi$ scattering lengths from measurement of $\pi^+\pi^-$ atom lifetime

The DIRAC experiment at CERN has achieved a sizeable production of + atoms and has significantly improved the precision on its lifetime determination. From a sample of 21227 atomic pairs, a 4% measurement of the S-wave scattering lengthdi erence ja0 a2j = 0:2533 +0:0080 0:0078 stat