## Chief

Yoshinobu Kuramashi, Professor He graduated from the Department of Physics, Graduate School of Science, the University of Tokyo, in 1995 with a Ph.D (Sciences). He assumed his present position after posts as an assistant professor at the Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization, and as a Lecturer and Associate Professor at the Center for Computational Sciences, University of Tsukuba. Concurrently, Dr. Kuramashi serves as the Field Theory Research Team Leader at the RIKEN Advanced Institute for Computational Science. |

## Strong interaction and Lattice QCD

Elementary particles have no internal structures. They are not composed of other particles. At present six flavors of quarks and six leptons are known as the elementary particles in the universe. They interact with each other via four fundamental interactions or forces: gravitation, electromagnetism, the weak interaction and the strong interaction. Among them the strong interaction, which acts between the quarks through exchanges of the gluons, shows peculiar features depending on the distance scale. As the distance between quarks decreases, the interaction strength becomes weaker so that the quarks are allowed to move more freely (asymptotic freedom). On the other hand, the interaction becomes so strong in the longer distance that the quarks are “confined” in the hadrons and never observed individually. The purpose of the Lattice QCD calculation is to prove that QCD is the fundamental theory of the strong interaction and investigate its dynamics nonperturbatively based on the first principles.

## Hadron spectrum from Lattice QCD

Large scale Lattice QCD simulations with the Monte Carlo method enable us to make quantitative predictions for various physical quantities. Our first target is to establish QCD as the fundamental theory of the strong interaction by reproducing basic physical quantities, e.g., the hadron spectrum. The right figure compares our recent results for the hadron spectrum, obtained by the use of the PACS-CS computer, with the experimental values. As seen from the figure, most of them are consistent within the error bars, though the worst cases show 2−3% deviations. We now go one step further and investigate a width of ρ→ππdecay.

Fig.1 Hadron mass spectrum

## Nuclear physics from Lattice QCD

A second target is to investigate the nuclear force acting between the nucleons, which has awaited a proper treatment based on the first principles since Yukawa proposed the meson theory in 1934. Recently we have successfully derived the potential between nucleons (the nuclear force) from Lattice QCD calculations. As shown in the figure below, the qualitative features of the nuclear potentials are reproduced. Furthermore we have succeeded in the first direct construction of the helium nuclei (^{4}He and ^{3}He) on the lattice. Although both are achieved at heavier quark masses than Nature and the so-called quenched approximation is employed for the latter case, it is clear that we now embark on a new era of multiscale physics with Lattice QCD, where the computational science can really explore and show its potential ability.

Fig.2 Potential Energy of Nucleon

### Web sites

Particle Physics Laboratory (Japanese)