First $\pi K$ atom lifetime and $\pi K$ scattering length measurements

The results of a search for hydrogen-like atoms consisting of ${\pi }^{\mp }{K}^{\pm }$ mesons are presented. Evidence for $\pi K$ atom production by 24 GeV/c protons from CERN PS interacting with a nickel target has been seen in terms of characteristic $\pi K$ pairs from their breakup in the same target ($178\pm 49$) and from Coulomb final state interaction ($653\pm 42$). Using these results the analysis yields a first value for the $\pi K$ atom lifetime of $\tau =(2.5_{-1.8}^{+3.0})$ fs and a first model-independent measurement of the S-wave isospin-odd $\pi K$ scattering length $\left|a_0^-\right|=\frac{1}{3}\left|a_{1/2}-a_{3/2}\right|= \left(0.11_{-0.04}^{+0.09} \right)M_{\pi}^{-1}$($a_I$forisospin$I$).