# Spectroscopy of n' mesic nuclei

The UA(1) problem is a long-standing question on the low-energy spectrum and dynamics of the pseudoscalar mesons in QCD. The η' meson is, in a naive picture, one of the Nambu-Goldstone bosons associated with the spontaneous breakdown of the U(3)L×U(3)R chiral symmetry to the UV(3) flavour symmetry. The gluon dynamics, however, plays an important role here. The quantum anomaly effect of non-perturbative gluon dynamics induces the QCD vacuum to be non-trivial, resulting the η' meson to acquire a peculiarly larger mass than other pseudoscalar mesons e.g. π, K, and η.

The mass generation of the η' meson is therefore a result of the interplay of quark symmetry and gluon dynamics but a quantitative understanding is yet missing.

An in-medium property of η' will give a vital constraint on the theory. At a finite density where chiral symmetry is partially restored, it is expected that the η' mass is reduced. The Nambu–Jona-Lasinio model suggests a mass reduction of 150 MeV/c2. This indicates that the interaction between a η' meson and a nucleus is attractive and that a η'-nucleus bound state may exist.

While there is no experimental information on the strength of the interaction nor the existence of the bound state, a small absorption width of η' at the normal nuclear density (15-25 MeV) is reported by CBELSA/TAPS.

The suggested narrow absorption width stimulates experimental studies of η' mesic nuclei.
We plan a missing-mass spectroscopy experiment at GSI by using a (p,d) reaction of a 12C- target. The fragment separator FRS will be used as a spectrometer.

The development of detector and data acquisition system are finalised. The performance of a special high refractive index Cherenkov detector, which is the key component to reduce the expected high background from quasi-free multi-pion production reaction: p+N→d+πs, has been proven to work as designed.