A theoretical physics research group of University of Tsukuba and University of Tokyo has successfully calculated the profile of nuclear force using the supercomputer at KEK. Their calculation reveals the presence of a hard core in nucleus for the first time using the numerical calculation of Quantum Chromodynamics (QCD). The result has been published on Physical Review Letters on its July 12, 2007 issue.
 
KEK's supercomputer system used in this research was installed in March 2006, which includes "IBM System Blue Gene Solution," and has peak performance of 57.3 TFlops. This system is one of the most powerful supercomputers in Japan, and is mainly used for theoretical research on particle and nuclear physics. KEK provides its machine time to the research proposals from university researchers which are adopted as the programs called "Large Scale Simulation Program."
 
The nuclear force, or "strong" force, acts on protons and neutrons in nuclei. This process is understood by QCD in terms of gluons acting on quarks, which is the ultimate constituent of matter. Due to a mathematically complicated nature of QCD, its exact behavior can only be obtained through numerical simulations of quark and gluon fields on a grid of four-dimensional space and time. Professor Sinya Aoki and Dr. Noriyoshi Ishii, University of Tsukuba, and Professor Tetsuo Hatsuda, University of Tokyo, for the first time succeeded in calculating the nuclear force in the full range of distances including the repulsive force. This research was made possible by using a special method calculating a pair of protons or neutrons interacting with each other. The result clearly shows the presence of repulsive core in proton and neutron.
 
"The strong repulsive core is essential for the stability of nuclei, the structure of neutron stars, and even the ignition mechanism of supernova," said Professor Aoki. "By further elaborating the simulation, we will be able to answer deeper questions, such as, why and how matters in the Universe exist in the present stable form."
 
The strong force, the force that binds nucleons together to form a nucleus, is mediated by pions, as predicted by Hideki Yukawa (1949 Nobel Laureate in Physics) in 1935. About 40 years later, more fundamental theory of strong interaction, QCD, was established. Both nucleons and pions are composite particles made of quarks bound together by gluons. In this present knowledge, the force between nucleons is much more complicated than originally thought by Yukawa. In fact, the attractive force between two nucleons must become repulsive at very short distances in order that they do not collapse into each other.
 
KEK's supercomputer has been utilized for many other fundamental QCD studies such as the symmetry breaking of the QCD vacuum.