Springer Theses

This book focuses on the study of the dynamical properties of hadron resonances, especially their transition processes by electromagnetic and strong interactions, by using the holographic quantum chromodynamics (QCD) model. Understanding the nature of hadrons leads to revealing non-perturbative phenomena that are prominent in the low-energy region of QCD. There remain many open questions regarding the nature of resonant states. Among the low-lying states of nucleons, the Roper resonance-the first excited state of nucleons-is the most controversial state even though it is the most well-established one. Holographic QCD is one of the methods to study hadrons taking into account the non-perturbative nature of QCD, where the top-down approach based on the D4-D8 brane construction of the so-called Sakai-Sugimoto model has successfully described physics of hadrons in light flavors. Using this model, the author provides a method to calculate the electromagnetic transition amplitude and the decay width of one pion emission for its low-lying states within the nucleon resonance. In particular, a systematic description of the Roper resonance, which is difficult to achieve by the conventional hadron effective model, is given. Furthermore, the author explains that the picture of resonant states as collective excitations rather than the conventional picture as single-particle excitations is critically important to comprehensively understand nucleon resonances. Recently, the existence of Roper-like resonant states has been established not only for light hadrons but also for heavy hadrons such as those containing charm and bottom quarks. Motivated by the fact, the author introduces heavy flavors into this model utilizing the extra-dimensional degrees of freedom, and successfully explains the method of dimensional reduction proposed by Forgacs-Manton, which is important for this purpose. The book contains a systematic review of the treatment of hadron physics by the Sakai-Sugimoto model. It also includes remarks on the modern understanding of hadron physics. The method of collective coordinate quantization of solitons-Skyrmion and Instanton-is also explained in a concise manner. These are useful not only for students and young researchers interested in this field.