In addition to the ordinary atoms, abnormal or exotic atoms serve as tools to study in detail the structure of atoms and their interactions. Different exotic atoms, consisting of other building blocks than electrons, protons and neutrons, have been observed and investigated.

The DIRAC collaboration at CERN has observed new kinds of exotic atoms, the “double-exotic” meson-meson atoms π+π– [A] as well as π–K+ and π+K– [B]. Furthermore, DIRAC determined their extremely short lifetimes (in the ground state) of around 10-15 s or 1 fs [C,D]. For analysing the atomic structure of these novel bound systems, DIRAC faces the difficult challenge to measure lifetimes of excited states.

The DIRAC experiment, a magnetic double arm spectrometer, succeeded in producing long-living π+π– atoms in a beryllium target at the CERN proton synchrotron (24 GeV) and to measure – for the first time – their lifetime of the 2p excited state. The generator/analyzer method of DIRAC is shown schematically in the below figure. The detected lifetime of ~5×10-12 s (5 ps) [E] is three orders of magnitude larger than the previously measured ground state lifetime ~3×10-15 s (3 fs), in agreement with the corresponding QED 2p lifetime.  

Further studies of long-living π+π– atoms will allow to determine the Lamb shift of this atom, i.e. the energy differences between p and s atomic states. By this means, crucial ππ scattering lengths can be evaluated and be compared with QCD predictions of strong interaction in the framework of Chiral Perturbation Theory and Lattice QCD.

References

[A] J. Phys. G30, 1929 (2004)

[B] Phys. Rev. Lett. 117, 112001 (2016)

[C] Phys. Lett. B 704, 24 (2011)

[D] Phys. Rev. D 96, 052002 (2017)

[E] Phys. Rev. Lett. 122, 082003 (2019)