2008 Nobel Prize in Physics to Profs. Makoto Kobayashi and Toshihide Maskawa
2009. 3. 3


The 2008 Nobel Prize in Physics was awarded to three Japanese theoretical particle physicists, Yoichiro Nambu, Toshihide Maskawa and Makoto Kobayashi. All of these physicists have deep connections with the institutions in which this GCOE is based, the Kyoto University Department of Physics and the Yukawa Institute for Theoretical Physics. We wish to express our joy and pride in this recognition of their work. We are particularly proud of the fact that the work for which Professor Kobayashi and Professor Maskawa have been cited was carried out when they were assistant professors in the Elementary Particle Theory Group headed by Professor Hideki Yukawa in our Department of Physics. Here I give a brief introduction to that work.

The theory constructed by Professors Kobayashi and Maskawa describes how and why so-called CP symmetry (a symmetry between particles and anti-particles) is broken. This theory plays a key role in the present standard model of elementary particle physics. At the time of the Big Bang, particles and anti-particles were produced in pairs, in exactly equal numbers. Then, if there had been exact CP symmetry, as the universe cooled, these particles and anti-particles would have been annihilated in pairs, and through this process, all of the matter in the universe would have disappeared, and our world never would have come to exist. Thus the existence of our world and all the matter we see is a result of the fact that CP symmetry is not exact. The small violation of CP symmetry was confirmed experimentally in 1964.

In 1972, after Weinberg and Salam published their work on the gauge theory unifying the electromagnetic and weak forces, Kobayashi and Maskawa began investigating whether the breaking of CP symmetry which appears in that theory could be explained. According the current theory of elementary particles, the fundamental particles in nature are quarks and leptons. As we now know, there are several hundred types of particles that interact through the strong interaction, for example protons, neutrons and pi mesons (predicted by Yukawa), and these in fact are not ``elementary" particles, but rather represent composite states of matter, formed from quarks. The particles that do note interact via the strong interaction, for example electrons and their neutrino partners, are referred to as leptons.

The electro-weak gauge theory in its formulation at that time was a theory regarding leptons, and it did not include quarks and the strong force through which they interact. The first accomplishment of the Kobayashi-Maskawa theory is that it elucidated the manner in which quarks enter this theory. In addition, it offered a new mechanism that can account for the breaking of CP symmetry. The basic idea of this mechanism is the following. Quark states can be expressed in terms of mass-eigenstates or weak-interaction eigenstates. It turns out that the these two sets of eigenstates do not coincide, but one set is obtained through the other through a small rotation. The unitary matrix that represents this rotation is known today as the Cabibbo-Kobayashi-Maskawa (CKM) matrix. This matrix, in part, describes the mechanism of CP violation. Kobayashi and Maskawa realized that in order for this mechanism to work, there had to be at least six types (six ``generations") of quarks, not just the three types [1] that were known to exist at the time. Thus, their theory predicts the existence of these additional quarks. At the time that Kobayashi announced this prediction, at a seminar of the Elementary Particle Group at Kyoto University, the members of the group, including myself, thought that while this work was interesting from the point of view of methodology, it was difficult to believe that its bold predictions were correct. However, soon after that time, a number of new quarks were discovered experimentally, and eventually, one by one, all six generations that their theory predicts were found, with the last, the top quark, being observed in 1995. In addition, highly precise experiments investigating CP violation through the study of B mesons conducted at the High Energy Accelerator in Tsukuba, Japan (the Belle Collaboration), and the Stanford University Linear Accelerator (the BaBar Collaboration), reported in 2001 and 2002, respectively, find no discrepancy with the predictions of the Kobayashi-Maskawa theory.

That these two scientists won the Nobel Prize is not only a source of great joy for the members of the Kyoto University Department of Physics and the Yukawa Institute, but it provides inspiration for the entire community of theoretical physicists in Japan. This is particularly true because it can be said that the Kobayashi-Maskawa theory was constructed on the rigorous tradition of the foundation of fundamental modern physics provided by Hideki Yukawa, Shinichiro Tomonaga and Shoichi Sakata. In fact, both Kobayashi and Maskawa are graduates of Nagoya University, where they studied under Sakata, and the Kobayashi-Maskawa theory is directly connected to the elementary particle composite models, including the quark model, that came out of the Sakata school of thought. In addition, they continued the tradition of Tomonaga by seeking to thoroughly investigate the logic of field theory, whose validity was, at that time, not entirely accepted. Finally, in can be said that they inherited the courage needed to predict the existence of yet undiscovered types of quarks from Yukawa.

Taichiro Kugo (Professor, Yukawa Institute for Theoretical Physics)

[1] In 1970, the experimental group of Kiyoshi Niu at Nagoya University discovered the fourth quark in cosmic-ray experiments, and as a result, the existence of this quark was believed among the physicists at Nagoya University. It may be the case that being educated in that environment made it easier for Kobayashi and Maskawa to conjecture the existence of six types of quarks.