Japanese High-Energy Accelerator Research Organization, KEK

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JAPANESE HIGH-ENERGY ACCELERATOR RESEARCH ORGANIZATION, KEK

The Japanese High-Energy Accelerator Research Organization (KEK) was established in 1971 by the Japanese government for the purpose of promoting experimental research in elementary particle physics. Although Japanese physicists were actively contributing to the theoretical developments in this field at that time, their experimental research activities were limited to cosmic ray observations even though high-energy particle accelerators had been the standard research tools since the 1950s.

The new laboratory's first mission was to build a proton synchrotron capable of accelerating protons up to an energy of 12 billion electron volts. One billion electron volts approximately corresponds to the energy needed to create one proton out of a vacuum. This accelerator began operating in 1975 and provided high-energy beams consisting of π mesons, K mesons, antiprotons, and protons for a wide range of particle physics experiments conducted for the first time in Japan. It was an important milestone for the development of particle physics in Japan.

Two more accelerators have been added to KEK's research facilities since then: the Photon Factory in 1982 and TRISTAN in 1986; the latter was then converted to a B Factory in 1999. The Photon Factory produces intense beams of light (or equivalently photons, thus the name Photon Factory) in the wavelength range stretching from ultravioletlight to X rays and has been used in research in material and biological sciences as well as for industrial applications. Accelerated electrons are stored in a circular orbit in this accelerator, and intense beams of light are generated from the circulating electrons.

In the 1980s, one of the most urgent issues in elementary particle physics was to find the top quark, the heaviest and only missing member among the theoretically proposed six-quark family. KEK joined this search by building a high-energy electron-positron colliding accelerator (called TRISTAN). In 1987, TRISTAN reached a collision energy of 64 billion electron volts, the highest electron and positron collision energy in the world at that time. The top quark was out of reach with the available energy, and the experimenters could only conclude that the top quark must be more massive than 32 billion electron-volts. When it was finally discovered at Fermi National Laboratory in 1995, the top quark turned out to have a mass of 174 billion electron volts.

In 1999, TRISTAN was converted to a new type of accelerator which generates particle-antiparticle pairs called B mesons and anti-B mesons. The B meson is an unstable particle, about five times heavier than a proton, and decays into several more stable particles immediately after being created. The anti- B meson is its antiparticle. Such a system can provide a laboratory for observing differences between particles and antiparticles, provided they can be generated in the millions.

Some important new findings were made as a result of KEK experiments. In 1989, a team from Japan, America, Korea, and China, working at a TRISTAN experiment, observed for the first time that gluon particles do interact among themselves. The gluon is a particle that carries the strong force between the quarks. Unlike the electromagnetic forces, where photons only

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carry the force between electrons and positrons but do not interact among themselves, gluons have been predicted to interact among themselves in addition to working as the carrier of strong force. Observation of this peculiar property of gluons was a welcomed experimental verification for the theory of strong force.

KEK developed a method of using the proton synchrotron as an intense source of neutrino particles, which are aimed at a large underground neutrino detector (called Super Kamiokande) located approximately 250 km away. Neutrinos, known to exist in three types, interact with other particles only very weakly. This elusiveness has prevented any experimental measurement of their masses in spite of the recognition that their tiny, if not zero, masses, can play a vital role in the evolving process of the universe. In 1999, a team of Japanese, Korean, and American scientists, counting the neutrinos entering their detectors at both the Super-Kamiokande and KEK sites, succeeded in detecting the neutrinos that traveled from KEK to Super-Kamiokande. This was the first time such measurements had been performed, and it opened the possibility of determining the neutrino masses using a phenomenon called "neutrino oscillation." Neutrinos are believed to go back and forth between different types, oscillating back and forth with a characteristic time frequency depending on their masses. Counting neutrinos that travel a long distance opens the possibility of measuring their oscillation frequencies.

In 2000, an international team of more than 200 scientists from eleven nations, working at the B Factory site, found convincing evidence that the B meson behaves differently from the anti-B meson, as seen in certain decay patterns. This result is an important step toward the comprehensive understanding of tiny and subtle differences between particles and antiparticles. In spite of many years of study, it has been difficult to pin down the origin of particle-antiparticle differences because they appear in only very limited processes. Whatever is causing these differences, a similar mechanism is believed to be responsible for the creation of our universe in its present form. In spite of a widely accepted belief that the Big Bang originally created equal amounts of particles and antiparticles, our universe is now completely dominated by matter (particles) with no trace of antimatter (antiparticles).

Located in Tsukuba Science City, 60 km north of Tokyo, KEK has evolved into a major research laboratory for elementary particle physics and other fields of science that use the particle accelerators as research tools. All the accelerator facilities are open to the international scientific community. Anyone or any team can use the KEK facilities as long as their research proposals are approved by a scientific committee of the laboratory. Quite often international collaborations are formed to face the scientific challenges more effectively. Besides providing a variety of high-energy particle beams to the experimenters, KEK leads advanced research and development toward building more powerful, next-generation particle accelerators.