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Special Topics

EPJ A Highlight - Pionic-Hydrogen Atom and Quantum Chromodynamics

Spectrum of the simultaneous measurement of the H(3p-1s) and the 16O(6h-5g) transitions (top). Energy shift of the ground state at various H2 density (solid diamonds), the open diamond represents the previous experiment when using the latest value of the electromagnetic transition energy.

Analogous to the vast amount of knowledge acquired on the electronic hydrogen atom over the last century and the success of Quantum Electrodynamics (QED), hadronic physics is using a similar system, namely “pionic hydrogen” - a hydrogen atom where the electron is replaced by a negatively charged pion - as a laboratory for investigating Quantum Chromodynamics (QCD). Like in electronic hydrogen the finite size of the proton plays a role in the precise determination of the ground state of the atom. The smaller Bohr radius of the pion offers a larger sensitivity to the strong interaction between the pion and the proton, leading, e.g., 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 pion-proton 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 a cyclotron trap and an ultimate resolution Bragg spectrometer leading to an impressive four fold improvement compared to the previous best measurement as shown in Fig. 1.

M. Hennebach et al. (2015), Hadronic shift in pionic hydrogen, European Physical Journal A 50: 190, DOI 10.1140/epja/i2014-14190-x

Managing Editors
Anne Ruimy (EDP Sciences) and Sabine Lehr (Springer-Verlag)
The collaboration for this special issue has been a pleasent experience.

Yong Zhou, Xiangtan University, China,
Editor EPJ Special Topics 222/8, 2013

ISSN: 1951-6355 (Print Edition)
ISSN: 1951-6401 (Electronic Edition)

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