Observation of $\pi^- K^+$ and $\pi^+ K^-$ atoms

The observation of hydrogen-like $\pi K$ atoms, consisting of $\pi^- K^+$ or $\pi^+ K^-$ mesons, is presented. The atoms have been produced by 24 GeV/$c$ protons from the CERN PS accelerator, interacting with platinum or nickel foil targets. The breakup (ionisation) of $\pi K$ atoms in the same targets yields characteristic $\pi K$ pairs, called "atomic pairs", with small relative momenta in the pair centre-of-mass system. The upgraded DIRAC experiment has observed $349\pm62$ such atomic $\pi K$ pairs, corresponding to a signal of 5.6 standard deviations.