Spectroscopy of "mesonic atoms"

The discovery of deeply-bound pionic states in heavy atoms at FRS has opened up a new field of fundamental studies of the meson-nucleus interactions, which contribute to the understanding of the non-trivial structure of the vacuum of quantum chromodynamics (QCD).
The experiments on the meson-nucleus bound system will first concentrate on the existence of the states and secondly will reveal possible modification of meson properties inside nuclear matter.
The results will help to answer the key question of partial restoration
of chiral symmetry breaking, which is related to the unknown process of mass evolution. The experiments employ transfer reactions with light incident nuclei like protons and deuterons and then look for bound states in missing mass spectra.
The high-momentum resolving power and the independent multiple-stage operation of the Super-FRS ion-optical system are essential key features for these experiments.
The first planned experiment is on η′ bound nuclei. The (p,d) reaction at 2,500 MeV is suitable for producing and observing η′-bound nuclei. The η′ meson has a peculiarly large mass compared with other mesons in the pseudo-scalar nonet. This large mass can be theoretically understood in terms of the chiral symmetry breaking and the UA(1) quantum anomaly of non-perturbative gluon dynamics which induces the non-trivial vacuum structure of QCD. One can study this quantum anomaly effect via spectroscopy experiments of η′ meson bound systems. The second candidates will be the η-bound nuclei. η-bound nuclei can be produced either by the (d,3He) reaction or by the (p,3He) reaction.