Evidence for $\pi$K atoms with DIRAC

Electromagnetically bound $π^{\pm}K^{\mp}$-pairs ($πK$-atoms) have been observed by the DIRAC-II experiment at the CERN-PS. The $πK$-atoms were produced by the 24 GeV/c proton beam impinging on a thin Pt-target. The $π^{\pm}$- and $K^{\mp}$-mesons from the atom dissociation were analyzed in a two-arm magnetic spectrometer. An enhancement was observed at low relative momentum, corresponding to the production of $173 \pm 54 πK$-atoms. From these first data a lower limit for the mean life of 0.8 fs was derived at the 90% confidence level. The mean life being related to the S-wave $πK$-scattering lengths $a_1$ and $a_3$ (isospin 1/2 and 3/2), one then obtains an upper limit for the difference $|a_1 −a_3|$ of $0.58 m^{−1}_π$ at the 90% confidence level.