Pionic Deuterium

The atoms formed after Coulomb capture of negatively charged pions rapidly evolve towards nuclear dimensions and eventually decay by strong interaction. The strong force affects energy and line width of X-rays emitted during a de-excitation cascade which allows precise measurements of hadronic quantities by means of crystal spectrometers. Low-energy QCD parameters can then be determined. The experimental accuracy has reached the per cent level or better, which coincides with recent theoretical achievements within the framework of effective field theories. (A.C.)