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Stanford Report, June 2, 1999

Fact Sheet: Asymmetric B Factory

The Asymmetric B Factory is a new high-energy physics facility built at the Stanford Linear Accelerator Center (SLAC). The $177-million project is a collaborative effort among SLAC, the Lawrence Berkeley National Laboratory and the Lawrence Livermore National Laboratory.

Known as a particle collider, the B Factory accelerates two beams of subatomic particles to nearly the speed of light. It then forces the beams to cross. At this intersection point some of the subatomic particles collide. Such collisions create other subatomic particles that help scientists better understand the fundamental principles underlying all matter.

Construction of the B Factory began in 1994 and ended in July 1998, when it began producing particle collisions. Funded by the U.S. Department of Energy, it has successfully completed its commission phase and has begun producing experimental data.

In parallel with the collider construction, an international collaboration of scientists and engineers built a 1,200-ton particle detector for the facility known as BaBar. The detector cost about $110 million, 40 percent of which came from foreign sources. More than 650 physicists and engineers from 73 institutions in nine countries have participated in its design and construction.

The B Factory is a major upgrade and conversion of an older SLAC collider known as PEP. A second ring of magnets, an improved microwave power system and a better vacuum system were added in the existing tunnel, which is more than a mile in circumference. Electrons circulate through the refurbished ring, while their antimatter counterparts ­ called "positrons" ­ circulate in the opposite direction inside the new ring. Collisions between the two beams occur at a crossover point, also known as the interaction point, which is surrounded by the detector.

What distinguishes the B Factory from previous colliders (and the reason it is called asymmetric) is that the electron beam energy differs from that of the positron beam. Past colliders have generated electron and positron beams of equal energy. The B Factory, by contrast, employs an electron beam with three times the energy of the positron beam. As a consequence, most of the new particles that are created are thrown forward in the direction of the electron beam. This feature is designed to allow scientists to pinpoint where the particles disintegrate and to measure their lifetimes and other properties more accurately than can be done at equal-energy colliders.

The B Factory is specifically engineered to produce millions of massive, short-lived subatomic particles called B mesons. During the past decade physicists around the world have recognized that B mesons provide the best opportunity to study a fundamental difference between matter and antimatter known as CP violation. This phenomenon, originally discovered in 1963, makes it slightly harder to transform matter into antimatter than vice versa. CP violation is thought to be a crucial part of the reason that there is essentially no antimatter remaining in the universe today.

Antimatter is identical to ordinary matter except that it has the opposite electrical charge. Electrons, for example, have a negative charge while their antiparticles, positrons, are positive; they both have exactly the same mass, however, and ­ apart from the difference in sign ­ behave the same way in electromagnetic fields. When a particle and its antiparticle meet, they completely annihilate each other in a flash of pure energy. This is what happens to the electrons and positrons that collide in the B Factory. Then, a fraction of an eye-blink later, the energy materializes as new subatomic particles.

For more than 30 years after its discovery in 1932, antimatter was thought to behave exactly like matter in every regard. But this similarity led to a major cosmological puzzle when the Big Bang became the accepted theory of the origin of the universe. If, as expected, equal amounts of matter and antimatter were created in the Big Bang, then why didn't they just annihilate each other to the point at which only energy remained? Why is there solid ground to stand on today?

The discovery of CP violation suggested a possible answer to this conundrum, by allowing a small excess of matter over antimatter to arise. That would mean that some matter had to remain after the annihilations ceased.

To determine whether this explanation is adequate, however, physicists need a much better and more complete understanding of CP violation. That understanding is what the B Factory is designed to provide.

High-quality photos and color graphics are available on the SLAC website at: or by called Michael Riordan at SLAC, (650) 926-2613. SR