2008.10.08
Nobel Prize: Outstanding Contribution to Particle Physics

Our warmest congratulation goes out to Makoto Kobayashi, Former Director of Institute for Particle and Nuclear Physics, KEK, Toshihide Maskawa, Professor emeritus of Kyoto University and Yoichiro Nambu, Professor emeritus of University of Chicago, for being awarded the 2008 Nobel Prize in Physics.

Professor Yoichiro Nambu introduced the concept of spontaneous symmetry breaking.

The first success of this theory was that it revealed the existence of a hidden symmetry behind the force among nucleons, which at that time seemed to be too complicated to describe theoretically. Nambu's theory explained the reason that the necleon acquires a mass by introducing a mechanism of broken symmetry. The presence of the light meson, now known as the pion, is the evidence that the hidden symmetry does actually exist and is broken spontaneously in the vacuum.

Later, the concept of spontaneous symmetry breaking turned out to be also essential in the Standard Model of elementary particles, that describes the fundamental forces among elementary particles. The origin of mass of particles is now understood to be a concequence of a broken symmetry in the Standard Model. The presence of the Higgs particle, that is predicted by the Standard Model, is now being tested by the Large Hadron Collider (LHC) experiments, for which KEK is significantly contributing.

Professor Nambu also proposed other innovative ideas in elementary particle physics. They include the formulation of string theory, the introduction of "color" degrees of freedom that binds quarks together to form a nucleon. In addition to his outstanding contribution to the theoretical elementary particles, he has also been contributing to the evolution of high energy physics in Japan, that we greatly appreciate. We cordially congratulate Professor Nambu on the award of the Nobel prize in physics.

Kobayashi and Maskawa proposed a mechanism for CP violation in the weak interaction in 1972. They pointed out that a tiny CP violation, a combination of charge and parity imbalance in neutral K meson decays could be attributed to the existence of six kinds of quarks, when only three quarks were known to exist. Their seminal work has become a major pillar of the standard model of elementary particles, and their landmark paper has been cited more than 5000 times by now. They directed their attention to the spontaneously broken gauge theory well before the theory was widely accepted. In relativistic quantum field theory, CPT symmetry, a combination of charge, parity and time, is automatically conserved, while other discrete symmetries, namely C and P symmetries, are maximally broken in the weak interaction. On the other hand, it has long been believed that the weak interaction respects CP, which is the symmetry under the exchange of particles and anti-particles, until a tiny CP violation was discovered in neutral K meson decays in 1964. Kobayashi and Maskawa investigated whether the CP violation can be understood in the context of the gauge theory of weak interaction, and found that a possible way out is to assume the existence of three more unknown quarks, namely charm quark and the ones in the third generation.

Discovery of the charm quark in 1974 was followed by the discovery of the bottom, the 5-th quark soon after in 1977. These established the three generation picture of the quark model. The 6-th top quark turned out to be extremely heavy in comparison to the rest of quarks and leptons, and was finally discovered in 1995 at the Tevatron collider in Fermilab. These successive discoveries of quarks have given strong support to the model proposed by Kobayashi and Maskawa.

The nature of CP violation can be precisely inspected by measuring the decays of the bottom quarks. At KEK in Japan, a B-factory was built in order to elucidate the mechanism of the CP violation in the standard model, under a hard competition with another B-factory being built at SLAC. In 2001, both SLAC and KEK convincingly showed that the CP symmetry is broken in the B meson system as predicted by the Kobayashi-Maskawa mechanism. This was followed by many other measurements of the B meson decays at the B factories. By now, these very precise measurements experimentally established the Kobayashi-Maskawa model.

We are extremely proud that Professors Y. Nambu, M. Kobayashi and T. Maskawa have made an outstanding contribution to the particle physics which fully deserves the Nobel Prize.

Atsuto Suzuki, KEK Director General