At present we perform research in the following two fields: The quark structure of hadrons, and the physics of many-body systems. These researches are performed in collaboration with Argonne National Laboratory (Physics Division), the University of Adelaide (CSSM) and RIKEN (RNC) . They were partially supported by a Grant of the Japanese Ministry of Education, Culture, Sports, Science, and Technology .
The quark structure of hadrons
The structure of the nucleon (proton or neutron) can be studied experimentally by high energy electron scattering. In the ''deep inelastic electron scattering'' process, the nucleon breaks up due to the scattering with the electron, and new hadrons are formed. The probability for this process (cross section) gives information on the momentum distribution of the quarks inside the nucleon. By studying the spin dependence of this scattering process, one can also extract information on the spin of the nucleon carried by the quarks. The structure of other hadrons, like pions, can also be studied by scattering experiments, and informations on the motion of the quarks inside the pion are obtained.
In order to study these processes, we use relativistic quark theories to describe the mesons and nucleons as two- and three quark bound states. We directly solved the relativistic two-body and three-body equations to obtain the wave functions for the mesons and nucleons.
Our recent results is the deep inelastic scattering (structure functions of the nucleon).
The problem whether the properties of a nucleon bound in the nucleus are different from the properties of a free nucleon is a very active field of present accelerator experiments and theory. We have studied the medium modifications of the nucleon structure functions in collaboration with the Jefferson National Laboratories, USA
. We solved the medium modifications of the structure functions (EMC effect)
, including our predictions for the spin-dependent structure functions. The measurement of the spin-dependent nuclear structure function is an important part of future programs at JLab.
The figure below shows our results for the nuclei 7
Li and 11
B. The blue lines show the familiar spin-independent EMC effect, which is compared to the experimental data. The red lines show our predictions for the spin-dependent (polarized) structure functions. Experimental confirmation would give important insights into in-medium quark dynamics. By using the same effective quark theory, we also studied the medium modification of the nucleon elastic form factors.
Recently, we are interested in transverse momentum dependent quark distribution functions (TMDs) and fragmentation functions (TMD FFs), which can describe the three-dimensional quark structure in momentum space. In a recent work, we have developed a theoretical framework for the multi-fragmentation processes including transverse momentum dependence
. Furthermore, we perform model calculations of the TMDs in the nucleon and mesons.
Many body systems
In our recent research on many body systems, we concentrated on the equation of state of nuclear matter at high baryon densities, the phase transition to quark matter, and the structure of compact stars.
In our previous works we constructed the nuclear matter equation of state by taking into account the quark substructure of the nucleons, and investigated the phase transition to color superconducting quark matter at high densities. One important result of these studies was that the long-standing problem of the "chiral collapse" of nuclear matter is solved when the confinement of quarks is taken into account.
we extended these studies to the case of isospin asymmetric matter in beta equilibrium, and used the resulting equation of state to investigate the structure of compact stars. The figure below shows an example of our results. The first figure shows the equation of state for pure nuclear matter (NM), for the transition to normal quark matter (NQM), and the transition to color superconducting quark matter (SQM). The second figure shows the resulting neutron star masses as functions of the central density, the third one shows the relation between the star masses and radii, and the fourth figure shows the profiles of the stars with the maximum mass. From the last figure, we see that the model allows for stable quark stars with radii of about 8km, where the SQM phase is realized within a radius of 6 km.
Another topic of our recent research on many-body systems is the collective rotational states in deformed nuclei. We investigated the so-called scissors mode in our first paper on this interesting subject.