Pionic-Hydrogen Atom and Quantum Chromodynamics (Vol. 46 No. 3)
Analogous to the vast amount of knowledge acquired on the electronic hydrogen atom over the last century as a probe of Quantum Electrodynamics, hadronic physics is using “pionic hydrogen” - a hydrogen atom where the electron is replaced by a negatively charged pion - as a laboratory for investigating Quantum Chromodynamics (QCD). The small Bohr radius of pionic hydrogen offers a large sensitivity to the strong pion-proton interaction, leading to an energy shift compared to the ground state energy if only the electromagnetic interaction is considered. The precise determination of this shift provides a benchmark of our understanding of the strong interaction from basic principles in QCD. To this end an exquisite experiment was devised and performed at the high intensity, low energy pion beam at the Paul Scherrer Institut using the cyclotron trap and an ultimate resolution Bragg spectrometer leading to an impressive four fold improvement compared to the previous best measurement as shown in the figure.
M. Hennebach et al. (+14 co-authors), Hadronic shift in pionic hydrogen, Eur. Phys. J. A 50, 190 (2014)