After having announced the statistically significant observation ($5.6\sigma$) of the new exotic $\pi K$ atom, the DIRAC experiment at the CERN proton synchrotron presents the measurement of the corresponding atom lifetime, based on the full $\pi K$ data sample: $\tau =(5.5_{-2.8}^{+5.0})\times {10}^{-15}\mathrm{s}$. By means of a precise relation ($\approx 1\%$) between atom lifetime and scattering length, the following value for the $S$-wave isospin-odd $\pi K$ scattering length $a_0^{-}=\frac{1}{3}(a_{1/2}-a_{3/2})$ has been derived: $|a_0^{-}|=(0.072_{-0.020}^{+0.031})M_{\pi }^{-1}$.

• Received 11 July 2017

DOI:https://doi.org/10.1103/PhysRevD.96.052002

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Published by the American Physical Society