We present evidence for the rst observation of electromagnetically bound π ± K ∓pairs (πK-atoms) with the DIRAC experiment at the CERN-PS. The πK-atoms are produced by the 24 GeV/c proton beam in a thin Pt-target and the π ± and K ∓mesons from the atom dissociation are analyzed in a two-arm magnetic spectrometer. The observed enhancement at low relative momentum corresponds to the production of 173 ± 54 πK-atoms. The mean life of πK-atoms is related to the s-wave πKscattering lengths, the measurement of which is the goal of the experiment. From these rst data we derive a lower limit for the mean life of 0.8 fs at 90% condence level.

The results of a search for hydrogen-like atoms consisting of π ∓ K ± mesons are presented. Evidence for π K atom production by 24 GeV/c protons from CERN PS interacting with a nickel target has been seen in terms of characteristic π K pairs from their breakup in the same target (178 ± 49) as well as in terms of produced π K atoms (653 ± 42). Using these results, the analysis yields a first value for the π K atom lifetime of τ = (2.5 +3.0 −1.8 ) fs and a first measurement of the S-wave isospin-odd π K scattering length |a − 0 | = 1 3 |a 1/2 − a 3/2 | = (0.11 +0.09 −0.04 )M −1 π (a I for isospin I).

The observation of hydrogenlike πK atoms, consisting of π^{-}K^{+} or π^{+}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 πK atoms in the same targets yields characteristic πK 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 πK pairs, corresponding to a signal of 5.6 standard deviations. This is the first statistically significant observation of the strange dimesonic πK atom.

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The DIRAC spectrometer installed at CERN PS was redesigned and upgraded in order to study simultaneously A ππ and A πK atoms, namely the bound states of π

could be achieved for the first time with a significance of more than 5 standard deviations [1]. On the basis of the same data sample, this paper presents the resulting πK atom lifetime and the corresponding πK scattering length.Using non-perturbative lattice QCD (LQCD), chiral perturbation theory (ChPT) and dispersive analysis, the S-wave ππ and πK scattering lengths were calculated. Swave ππ scattering lengths as described in QCD exploiting chiral SU (2) L ×SU (2) R symmetry breaking were confirmed experimentally at a level of about 4% [2-4]. These measurements -independently of their accuracy -cannot test QCD predictions in the strange sector based on chiral SU (3) L × SU (3) R symmetry breaking. However, this arXiv:1707.02184v2 [hep-ex]

The adapted DIRAC experiment at the CERN PS accelerator observed for the first time long-lived hydrogen-like π + π − atoms, produced by protons hitting a beryllium target. A part of these atoms crossed the gap of 96 mm and got broken up in the 2.1 µm thick platinum foil. Analysing the observed number of atomic pairs, n L A = 436 +157 −61 tot , the lifetime of the 2p state is found to be τ 2p = ( 0.45 +1.08 −0.30 tot ) · 10 −11 s, not contradicting the corresponding QED 2p state lifetime τ QED 2p = 1.17 · 10 −11 s. This lifetime value is three orders of magnitude larger than our previously measured value of the π + π − atom ground state lifetime τ = ( 3.15 +0.28 −0.26 tot ) · 10 −15 s. Further studies of long-lived π + π − atoms will allow to measure energy differences between p and s atomic states and so to determine ππ scattering lengths with the aim to check QCD predictions. (J.Schacher) 1 deceased cesses, these atoms are produced in s states distributed according to n −3 , n being the principal quantum number [8].The decay probability of short-lived π + π − atoms (A 2π , pionium) in s states is dominated (99.6%) by the annihilation process [9-13] π + + π − → π 0 + π 0 and is given by the ππ s-wave scattering lengths combination |a 0 − a 2 | (a I is the ππ scattering length for isospin I):

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