I would like to extend a hearty welcome to all participants to the 50th Anniversary Symposium of Kobayashi-Maskawa Model. I would also like to express my deep respect to the members of the organizing committee who have made tremendous efforts to make this symposium possible. Since the Kobayashi-Maskawa Model was first proposed in February 1973, it has been exactly 50 years since then. In this half a century, three more types of quarks were discovered in addition to the previously known three types, and it came to be thought that the CP non-conservation of quarks might be caused by this model. The fact that the CP problem is directly related to the six types of quarks was clearly proven in the 2000s by the results of the B-factories built in Japan and the US. Through this process, this model has now become part of the common knowledge of particle physics. I think the significance of this symposium is to remind ourselves of the background of the Kobayashi-Maskawa model and how it came to be a part of common knowledge. I was involved in the B factory experiment on the Japanese side, so I would like to talk a little about it from my perspective.

An important turning point for the Kobayashi-Maskawa paper, in addition to the discovery of heavy quarks, was a paper by Carter, Sanda, and Bigi around 1981. Here, according to the Kobayashi-Maskawa model, it was pointed out that the characteristic CP non-conservation should be observed in the time-dependent behavior of the B mesons. Since the lifetime and mixing of B mesons had been measured by earlier experiments, a discussion took place how the points made by Carter, Sanda, Bigi, and others could be experimentally verified, and experiments to make such measurements were proposed around the world. You all know of the B-factory experiments conducted at KEK and SLAC, but there were a number of experimental proposals in the 1980s using electron-positron collision by Cornell University, DESY, and PSI, and experiments using proton beam were BTeV at Fermilab and HERA-B at DESY. All of these experiments were designed to measure time-dependent behavior of B mesons, and I recall with nostalgia the lively discussions that took place at the workshops held around the world at that time.

The key to the B factory experiment at KEK was to achieve high luminosity and asymmetric collisions with electrons and positrons of different energies. The first proposal at KEK was to construct a small positron ring in the middle of the electron-positron collider called TRISTAN, which was in operation at the time, and collide it with electron beam of TRISTAN. However, Hirata and Keil pointed out that this idea would shorten the life of the beam, and it became clear that a major change was needed to match the circumference of the positron and electron rings. In the end, it was decided to proceed with the idea of installing both electron and positron accelerators in the 3-km-long TRISTAN tunnel, and it was realized thanks to the great efforts of then Director Sugawara and other leaders of KEK. By that time, the seven B experimental facilities proposed in the world had been organized, and construction finally proceeded simultaneously at SLAC and KEK.

At that time, I recall that many people criticized us, saying that SLAC, a first class research laboratory, and KEK, a second class laboratory, had no chance to compete with each other. In fact, the KEK accelerator team's ideas were not inferior to SLAC's, such as design of the collision point and the use of the ARES cavity to reduce beam instability. However, it was still very difficult to bring the luminosity up to the design value. One of the things I remember most vividly was the work we all did together to wind coil on the positron ring over a circumference of 3 km to reduce the phenomenon known as electron cloud instability. The effect was tremendous, and we were able to make dramatic progress in luminosity.

The particle spectrometer was built by the Belle collaboration. I think the design concept here was to create something that worked reliably without being too much advanced or high-performance detector. This was in contrast to SLAC's BaBar detector, but I don't think the performance of the instrument compromised the results at all. Rather, I believe that the Belle produced cleaner experiments that were not affected by the background created by the high-current beam.

In 2000, a major international conference called ICHEP was held in Osaka, and BaBar and Belle presented their first results at the same time. In 2008, Professors Kobayashi and Maskawa were awarded the Nobel Prize in Physics, and I was pleased to shake hands with the BaBar representatives who were also invited to the award ceremony. Since then, we have accumulated more data and measured the unitarity triangles with high precision, and it is now certain that quark mixing and CP non-conservation can be understood by the Kobayashi and Maskawa model.

Flavor physics using electron-positron collisions has been taken over by the next generation experiment, Belle II at SuperKEKB. Although we have not yet accumulated enough data to produce full-scale results, we are confident that detailed measurements of B, D, τ, etc., in the future may well reveal the beginnings of new physics. For the time being, I believe that the most important tasks are to increase the luminosity of the accelerator sufficiently and to develop a strategy for the next generation of flavor physics. Of course, how to secure operating expenses in the face of rising electricity costs around the world is also a very important issue. We will do our utmost to find a solution to this problem. I believe that the cooperation among accelerator physicists, experimentalists, theorists, and us will one day lead to great results. I wish you all the best for this symposium. Thank you.