Spectrum and Decays of Hadronic Atoms

Hadronic atoms are highly non-relativistic loosely bound systems of hadrons mainly formed by the Coulomb interaction. The study of these hadronic bound states has seen growing interest during recent years, because they provide important information on the behavior of QCD at low-energy. The DIRAC collaboration 1 at CERN measures the lifetime of the ππ atom, and plans to determine the ππ scattering lengths difference |a0 − a0| at 5% accuracy. Particularly exiting is the fact that in this manner the current understanding of the SU(2)×SU(2) chiral symmetry breaking in QCD can be verified experimentally. Another experiment on hadronic atoms is presently performed at PSI. The pionic hydrogen collaboration 2 plans to measure the strong energy shift and decay width of the πp atom in its ground-state at the 1% level. This high precision measurement aims to determine the S-wave πN scattering lengths. Finally, the DEAR collaboration 3 at the DAΦNE facility measures the strong energy shift and width of the ground-state in kaonic hydrogen. Here, the goal is to extract the KN S-wave scattering lengths. In order to extract the scattering lengths from such precision measurements, the theoretical expressions for the spectrum and decays must be known to a precision that matches the experimental accuracy. Nearly fifty years ago, Deser et al. 4