The Research Center for Electron Photon Science (ELPH), Tohoku University, is a university-based electron accelerator facility to provide high-intensity GeV-bremsstrahlung-photon beams. We have studied hadron production and interactions utilizing meson photoproduction experiments with these beams. The recent activities at ELPH are overviewed: studies of dibaryon states observed in coherent double neutral-meson photoproduction on the deuteron, and those of the low-energy hadron-hadron scatterings from the final-state interaction.

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.