Spectrum and decays of kaonic hydrogen.

By using the non-relativistic effective Lagrangian approach to bound states, a complete expression for the isospin-breaking correction to the energy levels and the decay widths of kaonic hydrogen is obtained up-to-and-including (Oα,md-mu) in QCD. It is demonstrated that, although the leading-order corrections at O(α½,(md-mu)½) emerging due to the unitarity cusp are huge, they can be expressed solely in terms of the K N S-wave scattering lengths. Consequently, at leading order, it is possible to derive parameter-free modified. Deser-type relations, which can be used to extract the scattering lengths from the hadronic atom data. [ABSTRACT FROM AUTHOR]