Hadronic molecules are the new form of matter induced by the strong interaction. However, identification of the hadronic molecules involves several subtle difficulties such as the model dependence and the interpretation of the resonance wave function. To overcome these difficulties, we use the compositeness to characterize the internal structure of hadrons, and generalize the weak-binding relation for unstable resonances. It is quantitatively shown that the structure of the Lambda(1405) resonance is dominated by the molecular state of an antikaon and a nucleon.

Resonance phenomena commonly appear in various fields in physics. In particular, most of hadronic particles should be treated as resonances, because they are unstable against the strong decay. These lectures introduce the basic concepts of resonance physics. We then discuss the techniques to study the structure of resonances, aiming at the applications in hadron physics. The topics include: resonances in quantum mechanics, resonances in scattering theory, theory of Feshbach resonances, effective field theories, and compositeness of resonances.

In 2013, the BESIII collaboration in China and the Belle collaboration in Japan reported the observations of four-quark states. In this seminar, the constituent quark model and the diquark are introduced. Some structure and dynamics of the baryons, four-quark states and pentaquark states in terms of diquark are reviewed. The candidates of four-quark states and pentaquark states are given and some explanations in the normal quark model are given. In particular, the diquark cluster is examined in the strong decays of baryons with one heavy quark in the 3P0 model.

Possible existence of η' meson-nucleus bound states (η'-mesic nuclei) has recently attracted both theoretical and experimental interests, since their properties such as binding energies and widths would reflect nature of axial U(1) anomaly and chiral symmetry breaking in Quantum Chromodynamics (QCD). We started experimental programs to investigate η'-mesic nuclei at GSI Heavy Ion Research Center (Germany). The first experiment was performed in 2014 by using a 2.5 GeV proton beam to produce η'-mesic nuclei via the ¹²C(p,d)η’x¹¹C reaction. The excitation energy of ¹¹C was obtained by measuring the momentum of the outgoing deuteron with the high-resolution spectrometer FRS. Despite the achieved high energy resolution and statistical sensitivity, no peak structure indicating the formation of η’-mesic nuclei was observed. Thus, upper limit for the formation cross section was determined and a constraint on the η-nucleus interaction was deduced.
Presently, we are preparing for a future experiment with further improved experimental sensitivity. The key of this new experiment is simultaneous measurements of the forward outgoing deuteron from the ¹²C(p,d)η’x¹¹C reaction and decay particles (protons) from η’-mesic nuclei. This semi-exclusive measurement will allow us to efficiently select signal events and thus suppress a large amount of physical background processes. The experiment is feasible by combining the FRS (or Super-FRS) spectrometer at GSI (FAIR) in Germany with a large acceptance detector system (WASA) surrounding a reaction target. Various developments are ongoing to realize this new experimental setup by the end of 2020 and to start production runs in 2021.
In this seminar, we will present the results of the first experiment and discuss the proposed future experiment. We will also introduce ongoing activities and developments to integrate the WASA detector system to FRS at GSI.