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We present evidence for the rst observation of electromagnetically bound π ± K ∓pairs (πK-atoms) with the DIRAC experiment at the CERN-PS. The πK-atoms are produced by the 24 GeV/c proton beam in a thin Pt-target and the π ± and K ∓mesons from the atom dissociation are analyzed in a two-arm magnetic spectrometer. The observed enhancement at low relative momentum corresponds to the production of 173 ± 54 πK-atoms. The mean life of πK-atoms is related to the s-wave πKscattering lengths, the measurement of which is the goal of the experiment. From these rst data we derive a lower limit for the mean life of 0.8 fs at 90% condence level.

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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.