It is an ultimate theame of physics to build the unified theory of fundamental interaction including gravity. As the energy frontier of physics is getting higher, the compatibility with the cosmic evolution has become one important mean to test various models. The chemical abundance generated in the early universe is especially one of the most important touchstones.
In this field, Dr. Masahiro Kawasaki, Dr. Kazunori Kohri, and Dr. Takeo Moroi have achieved collaborative works that have gained worldwide reputation as mentioned below. Many particles that become unstable only through gravitational interaction, such as gravitino, exist in the supergravity theory, which is thought to be a candidate of the unified theory of fundamental forces including gravity, Such particles have a long life time and may eventually decay at the time of nuclear synthesis in the early universe, affecting the resultant chemical composition of light elements. It is possible, therefore, to place severe constraint on the properties of these long-life particles from the conditions not to spoil the nucleosynthesis. When gravitinos decay mainly in radiative modes, the deviation from standard chemical composition arises due to the effect of destroying light elements.
Dr. Kawasaki and Dr. Moroi analyzed the spectrum of the radiative fields by carrying out numerical integration of the Boltzmann equation, and they obtained a tighter constraint on the gravitino mass than what had been previously derived.
Dr. Kohri joined this research and brought considerable progress in the research on the modes in which unstable particles, such as gravitino, decay into hadrons. The effects of hadronic decay had not been investigated sufficiently at that time because of the difficulty in handling QCD jets emitted at the time of decay. They darely tackled this problem, and calculated the chemical composition correctly taking into account hadronic decay channels for the first time in the world.
They obtained very severe constraints on the abundances of gravitino and other particles. Furthermore, they also conducted comprehensive analysis including scenarios in which the gravitino is stable.
This series of research is really suitable for this prize in that it gives the foundation for the research that derives constraints on particle physics models through the nucleosynthesis in the early universe. In addition, these three persons individually have established important achievements besides this joint research, and it is clear that they continue to act as leading researchers in the field of particle cosmology. For these reasons, it was determined to send 7th Yukawa-Kimura prize to Dr. Kawasaki, Dr. Kohri, and Dr. Moroi.