Evidence for πK-atoms with DIRAC

Abstract We present evidence for the first 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 πK - scattering lengths, the measurement of which is the goal of the experiment. From these first data we derive a lower limit for the mean life of 0.8 fs at 90% confidence level.