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 $|a_0-a_2| = (.0.2533^{+0.0080}_{-0.0078}|_\mathrm{stat}.{}^{+0.0078}_{-0.0073}|_\mathrm{syst})M_{\pi^+}^{-1}$ has been attained, providing an important test of Chiral Perturbation Theory. 
B. Adeva, L. Afanasyev, M. Benayoun, A. Benelli, Z. Berka, V.   Brekhovskikh, G. Caragheorgheopol, T. Cechak, M. Chiba, P.V....
hep-ex hep-ph