Study of Hadronic Hydrogen-like Atoms in the DIRAC Experiment at PS CERN

The DIRAC experiment aims to observe and study hydrogen-like atoms formed by pairs of π+π− (A2π) and π±K∓ (AπK) mesons using the 24 GeV extracted beam of PS CERN. The lifetime of these atoms is dictated by the strong interaction between the components. Thus combining hadrons into a hydrogen-like atom opens a unique possibility to study a property of the strong interaction at the very low relative momenta which are of the order of the atom Bohr momentum. For π+π− atom it is 0.5 MeV, for πK — 0.8 MeV. Hence, the region of QCD confinement becomes available for investigation. The ground-state lifetime of π+π− atom τ2π is governed by the ππ S-wave scattering lengths aI , with isospin I = 0, 2 [1–3]: 1/τ2π ∝ |a0− a2|. For the πK atom, the lifetime τπK depends on the πK S-wave scattering lengths with isospin 1/2 and 3/2 [4]: 1/τπK ∝ |a1/2 − a3/2|. The values of these scattering lengths can be rigorously calculated in the Chiral Perturbation Theory (ChPT) [5, 6]. Last results of Lattice QCD [7] confirm this calculation. Thus, the measurement of the hadronic hydrogen-like atom lifetimes provides an experimental test of the low-energy QCD predictions.