Hyperfine structure in pionic atoms: relativistic calculation

We present the hyperfine structure (HFS) energies calculation for pionic atoms. These systems are formed of an atomic nucleus and of negatively charged pion (a spin-0 particle) replacing the electrons. The evaluation of the HFS sub-level energies are essential to estimate the broadening and mean value shift in the experimental atomic lines due to the HFS. The formulas are developed in a completely relativistic framework, using the multipole expansion of the nuclear potential in the Klein-Gordon equation. (author)