Observation of π^{-}K^{+} and π^{+}K^{-} Atoms.

The observation of hydrogenlike pi K atoms, consisting of pi K--(+) or pi K-+(-) mesons, is presented. The atoms are produced by 24 GeV/c protons from the CERN PS accelerator, interacting with platinum or nickel foil targets. The breakup (ionization) of pi K atoms in the same targets yields characteristic pi K pairs, called "atomic pairs," with small relative momenta Q in the pair center-of-mass system. The upgraded DIRAC experiment observed 349 +/- 62 such atomic pi K pairs, corresponding to a signal of 5.6 standard deviations. This is the first statistically significant observation of the strange dimesonic pi K atom.