A perspectuve on Structure of the nucleon

초록

The nucleon, though it is the most important constituent of matter, has an extremely complicated structure. However, recent advancement of experimental and theoretical works reveals a colorful structure of the nucleon. For example, as viewed from a light front moving towards a transversely polarized nucleon, we can map out the transverse charge density in the transverse plane. In this way, we can better understand how the charge of the quarks is distributed inside the nucleon, how their spins are transversely polarized, etc. Since lattice QCD yields many interesting results for nucleon observables that are difficult to be measured experimentally, we can investigate various faucets of the nucleon structure. In the present talk, we review a series of recent works on the structure of the nucleon, based on the chiral quark-soliton model both in free space and in nuclear matter. Firstly, we discuss the transverse charge densities of the quarks inside a nucleon. When the nucleon is polarized, the charge and magnetic distributions of the quarks show interesting feature. We also present the transverse spin structure of the nucleon, for which the tensor form factors of the nucleon play an essential role. Secondly, the medium effects on the nucleon are discussed. The energy-momentum tensor (EMT) form factors are important nucleon observables, since they have information on the mass and pressure of the nucleon. The null pressure of any hadron is required to establish its stability as a particle. Thus, the EMT form factors of the nucleon in medium are as important as other form factors such as the electromagnetic ones. While quark confinement does not come into play when it comes to the ground-state nucleon, it has crucial effects on excited nucleons and hyperons. An excited valence quark in the nucleon has a large kinetic energy, it is inevitable to consider the physics of the quark confinement to describe the nucleon, the excited valence quark being kept in it.