Hadron theory. Internal structure of hadrons and nuclei. Spin structure of the proton.
I have been investigating structure of hadrons and nuclei in terms of quarks and gluons. In particular, parton-distribution functions (PDFs) for the nucleon and nuclei are studied in order to establish quantum chromodynamics (QCD) and to find new phenomena beyond the standard model and new hadron-quark matters in high-energy hadron reactions.
Proton spin is one of fundamental physical quantities, yet its origin is not clarified. It was found that quark and antiquark spins carry only a small fraction of the proton spin, so that gluon spin or orbital angular momenta should be investigated as a candidate. I have been investigating this issue theoretically by using perturbative QCD and by determining the polarized PDFs from experimental measurements for spin asymmetries.

Hadron Physics.
Properties and interactions of hadrons.
Hadrons and hadronic matter at finite temperature and/or density.

I am interested in hadron physics in extreme conditions, such as in very high energy scatterings, and at very high densities or temperatures. Our standard knowledge about a nucleon that it is made of three quarks is in fact not useful when we consider the scattering of nucleons at very high energies, because the nucleon at high energy behaves like a dense matter of gluons (another constituents of a nucleon). This new form of matter, called "Color Glass Condensate" is by itself intriguing from theoretical point of view, but there are accumulating evidences that the Color Glass Condensate is already seen in experiments. Moreover, the Color Glass Condensate provides an initial condition for the relativistic heavy-ion collisions, and thus becomes one of the important ingredients for understanding properties of the quark gluon plasma, which will be created in the heavy ion collisions.

Light nuclei have various structures. They are very interesting because their structure changes drastically as proton and neutron numbers change. For example, the shape of a nucleus changes from spherical one to elliptic one, and a nucleus is separated to some units (clustering structure).

Recently I am attracted by light nuclei including hyperons ( Λ, Σ and Ξ) and K- mesons which contain a strange quark. They are called hyper nuclei and kaonic nuclei, respectively. These hyper nuclei and kaonic nuclei are considered to have so exotic properties that we have never seen in the study of conventional nuclei. Especially, the kaonic nuclei that I am now concentrating on are very exotic. The interaction between a K- meson and a nucleon (proton and neutron) is strongly attractive. As a result that the K- meson attracts nucleons around itself, it is expected that strange structures appear and highly dense state is formed in the kaonic nuclei. Such a dense state formed in a nucleus is important also in the hadron physics. Now, I am studying the kaonic nuclei from such a viewpoint.