Muonic atom cascade program

Abstract

The new era in X-ray spectroscopy of exotic atoms is based on high-resolution reflection-type crystal spectrometers, state-of-the-art X-ray detectors, and sophisticated set-ups to stop the negatively charged particles provided by modern accelerator facilities. Measurements on the elementary systems formed with hydrogen and helium isotopes yield a precision unprecedented in low-energy strong-interaction physics. Spin–spin and spin–orbit effects were identified in antiprotonic hydrogen and hadronic effects were observed for the first time in antiprotonic deuterium. In kaonic hydrogen strong-interaction effects could finally be identified unambiguously. For the pion–proton and pion–deuteron system the measurements reach an accuracy for the hadronic shift of a few per mill, which demands further theoretical effort to extract the scattering lengths at the same level. To allow a precise determination of the pion–nucleon coupling constant, which constitutes a stringent test of the approach for quantum chromodynamics in the non-perturbative regime, a new series of measurements has been started aiming at an accuracy of 1% for the hadronic width in pionic hydrogen. The mass of the charged pion was re-measured by using light pionic and muonic atoms and the first direct observation of Coulomb explosion was achieved for exotic atoms. Tests of bound-state quantum electrodynamics became possible at an accuracy which in turn can be used now to establish X-ray standards in the few keV range by the pionic atoms themselves.