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

The DIRAC experiment at CERN has achieved a sizeable production of ${\pi }^{+}{\pi }^{-}$ 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 $\pi \pi$ scattering length difference $|a_0-a_2| = (.0.2533^{+0.0080}{-0.0078}|$\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}$.{}^{+0.0078}{-0.0073}|$\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}$)M_{\pi^+}^{-1}$ has been attained, providing an important test of Chiral Perturbation Theory.