Detection of π+π− atoms with the DIRAC spectrometer at CERN

The goal of the DIRAC experiment at CERN is to measure with high precision the lifetime of the π + π - atom (A 2π ), which is of order 3 x 10 -15 s, and thus to determine the s-waveππ- scattering lengthsdifference |a 0 - a 2 |. A 2π atoms are detected through the characteristic features of π + π - pairs from the atom break-up(ionization) in the target. We report on a first high statistics atomic data sample obtained from p-Ni interactions at 24 GeV/c proton momentum and present the methods to separate the signal from the background.