Beijing Accelerator Laboratory

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BEIJING ACCELERATOR LABORATORY

Beijing Electron-Positron Collider

The Beijing Electron-Positron Collider (BEPC) is located 12 kilometers west of the center of Beijing, China, at the Institute for High Energy Physics. The purpose of the laboratory is to conduct research in elementary particle physics and to serve as a source of synchrotron radiation, which is a special form of light created when electrons' paths bend in a strong magnetic field. The laboratory is composed of three main components: a collider (BEPC), a magnetic spectrometer (Beijing Spectrometer [BES]), and synchrotron radiation facilities.

In a collider, bunches of electrons and positrons circle in opposite directions in an evacuated tube, shaped like a doughnut, and are held in their orbits by strong magnetic fields. They can collide head-on, annihilate, and form new particles. Since 1990, BEPC has been the only collider in the world operating in the 2 to 5 billion electron volt energy range. This is a very interesting energy region because this is the threshold region for the production of tau leptons (heavy versions of the electron) and many particles made from charmed quarks.

Origin

The 1970s were an exciting period in high-energy physics. The J/ψ particle had been discovered simultaneously in 1974 in different experiments by groups led by Burton Richter at the Stanford Linear Accelerator Center (SLAC) in Palo Alto, California, and by Chinese-American Samuel C. C. Ting at the Brookhaven National Laboratory in Long Island, New York. The discovery at SLAC was made in a new electron-positron collider, called SPEAR. The J/ψ particle, more than three times heavier than a proton, is composed of a charmed quark and an anticharmed quark held together by the strong nuclear force. Richter and Ting shared the Nobel Prize in Physics in 1976 for their discoveries.

In 1981, the director of SLAC, Wolfgang K. H. Panofsky, and Chinese-American Nobel Prize winner Tsung Dao Lee suggested that China build a 5 billion electron volt collider. This suggestion received strong support from Chinese physicists, and a proposal was made to build a machine with a similar energy range as SPEAR but with higher interaction rates, allowing more detailed studies in this region. Premier Deng Xiaoping officially approved the BEPC project in 1982 and attended the groundbreaking on October 7, 1984. After four years, on October 22, 1988, he attended the celebration of the first electron-positron collisions.

Beijing Electron-Positron Collider

Figure 1 shows the BEPC accelerator complex. The straight section on the right is the injector, which is a 202-meter-long linear accelerator. An electron gun, which is the source of electrons, is at the beginning of the injector. Electrons are accelerated to an energy of 1.1 to 1.4 billion electron volts and are transferred into the storage ring on the left. The storage ring has a circumference of 240 meters and is shaped like a racetrack. To make positrons, electrons strike a moveable tungsten target located partway down the linear accelerator and create both positrons and electrons. Some of the positrons formed are accelerated through the remaining portion of the linear accelerator and injected in the opposite direction into the collider. The beams are separated vertically except where they collide at the interaction point.

The Beijing Spectrometer Detector

Many different kinds of particles can be formed in the collisions. In most cases, the particles exist for a very short time before they decay into other longer-lived particles. To record what happens, a large magnetic detector named the Beijing Spectrometer (BES) is used. Its main function is to measure the parameters of the particles and to identify them. BES is cylindrical in shape, approximately 4.7 meters long and 4.9 meters in diameter, and is composed of many subsystems. Close to the beam pipe is a vertex detector to measure the positions of the charged particles passing through it. Next is the main drift chamber, which measures the trajectories of charged particles. Because the chamber is located in a large magnetic field, the trajectories curve, and the amount of curvature determines the momentum of each particle.

Particles next pass through the time of flight system, which determines the time of travel from the interaction point for each particle and thereby the particle's velocity. By measuring the velocity and momentum, the mass and type of particle can be determined.

Further out is the shower counter, which measures the energies of photons (particles of light) coming from the interaction. Photons are electrically neutral and are not detected in the main drift chamber. Outside the shower counter is the coil of the magnet that supplies the magnetic field in BES. All the signals from the various subsystems of BES are read into a computer using special electronics and are written to tape for later analysis.

Physics Results

BES began taking data in 1990. In 1991, physicists from the United States joined the effort, making the BES experiment an international collaboration. In 2001, the collaboration included about 150 physicists from China, Japan, Korea, and the United States.

BES has published papers in English and Chinese journals on many physics topics. One of the most important measurements was the measurement of the mass of the tau lepton. In the early 1990s, the values of the tau mass and tau lifetime, measured by previous experiments, implied disagreement with the theory of lepton universality, part of the Standard Model of high-energy physics. According to this theory, all leptons (electrons, muons, and taus) should have the same behavior.

In 1992, BES measured the tau mass with a precision of 0.02 percent by carefully measuring the tau production rate near threshold. The measured value was

FIGURE 1

lower than the previous value, bringing it into very good agreement with the theory of lepton universality.

In 1998 and 1999, BES measured the electron-positron interaction rate at ninety-one different energies between 2 and 5 billion electron volts. The uncertainties on these values were less than one-half the previous ones. These improved measurements are important for precision tests of the Standard Model and have been used to improve the predicted mass of the Higgs meson.