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 $\\pi^+\\pi^-$ atom ($A_{2\\pi}$), which is of order $3\\times10^{-15}$ s, and thus to determine the s-wave $\\pi\\pi$-scattering lengths difference $|a_{0}-a_{2}|$. $A_{2\\pi}$ atoms are detected through the characteristic features of $\\pi^+\\pi^-$ 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.