## QCD with light quarks

### Kaonic Atoms

Precision X-ray spectroscopy of kaonic atoms represents an excellent tool to study the chiral symmetry breaking scenario involving a strange quark. Kaonic atoms are QED bound systems in which the heavier, negatively charged particle replaces an electron, for example a kaonic hydrogen (K^{-}p) atom could be formed. In general, studies of exotic mesonic atoms have provided important information on strong interaction (hadron) physics.

Effective field theories (EFTs) provide a crucial framework for analysing the properties and interactions of hadrons and nuclei. These theories describe low-energy hadron physics implementing the symmetries of the underlying theory, QCD, using effective Lagrangians for the relevant degrees of freedom, which leads to observable effects in the spectrum of exotic hadronic atoms. [Read more...]

### Hadronic Systems with Strangeness

Strangeness hadron physics tries to reveal the complex dynamics and phenomena of quarks and gluons, e.g. hadron properties in nuclear medium, symmetry breaking pattern and hadron mass generation, new forms of hadrons.

While for short distances (high energy) the "strong" QCD interaction is characterised by colour confinement and asymptotic freedom, the interaction becomes weak and can be treated perturbatively. On the contrary, at long distances (at the temperature of the present Universe) the interaction stays strong and colour charges are confined. The chiral symmetry and the heavy quark symmetry play a role in the QCD at the light quark sector (m_{u}, m_{d} << Λ_{QCD}) and at the heavy quark sector (m_{c}, m_{b}, m_{t} >>Λ_{QCD}), respectively. At the energy scale of the strange quark mass (≈Λ_{QCD}), however, neither of the symmetries is good enough and the dynamics of the strange quark is thus very sensitive to the dynamics of QCD. [Read more...]

### Spectroscopy of η' mesic nuclei

The U_{A}(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 U_{V}(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. [Read more...]