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

Abstract

Production of hadronic hydrogen-like atoms at high-energy collisions and method of their observation are considered. Main results of the DIRAC experiment on observation and lifetime measurement of atoms formed by pairs of charged pion-pion and pion-kaon are presented. Physics of hadronic hydrogen-like atomsThe DIRAC experiment aims to observe and study hydrogen-like atoms formed by pairs of π + π − (A 2π ) and π ± K ∓ (A πK ) 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 τ 2π is governed by the ππ S-wave scattering lengths a I , with isospin I = 0, 2 [1-3]: 1/τ 2π ∝ |a 0 − a 2 | 2 . For the πK atom, the lifetime τ πK depends on the πK S-wave scattering lengths with isospin 1/2 and 3/2 [4]: 1/τ πK ∝ |a 1/2 − a 3/2 | 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.