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Abstract. 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 3×10 −15 s, and thus to determine the s-wave ππ-scattering lengths difference |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.

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The DIRAC spectrometer has been commissioned at CERN with the aim of detecting π + π − atoms produced by a 24 GeV/c high intensity proton beam in thin foil targets. A challenging apparatus is required to cope with the high interaction rates involved, the triggering of pion pairs with very low relative momentum, and the measurement of the latter with resolution around 0.6 MeV/c. The general characteristics of the apparatus are explained and each part is described in some detail. The main features of the trigger system, data-acquisition, monitoring and setup performances are also given.

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