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Particle physics: Stanford wins a B Factory.

Mothball since 1990, the Positron-Electron Project (PEP) particle accelerator at the Stanford Linear Accelerator Center (SLAC) had outlived its usefulness. But last week, its fortunes were revived when the Department of Energy selected SLAC as the site of a new research facility known as the B Factory.

Designed to mass-produce large quantities of subatomic particles known as B mesons via collisions between electrons and positrons (the oppositely charged, antimatter counterparts of electrons), the B Factory will be installed in the existing tunnel occupied by the PEP colliding-beam storage ring.

"This facility will be a crucial element in a balanced U.S. high-energy physics program and will ensure continued U.S. leadership on the electron frontier," says Burton Richter, SLAC director.

Construction of the B Factory would allow particle physicists to study in great detail a subatomic process -- known as CP violation -- that may be responsible for the overwhelming perponderance of matter over antimatter in the universe. Theorists suggest that the universe started out with equal amounts of matter and antimatter and that somehow a minuscule anomaly at the very beginning skewed this distribution toward matter.

Discovered in 1977, B mesons consists of a bottom quark paired with an anti-up or an andi-down quark. When these particles decay into other particles, a small fraction doesn't follow the usual rule that leaves parity, a characteristic of particle interactions, unchanged. Thus, it's possible in these interactions to distinguish between our world and its mirror image, even when particles are replaced by their antiparticles. By measuring the decays of large numbers of B mesons, physicists can study these rare, CP-violating events more readily.

In the B Factory, collisions between electrons and positrons in beams having a total energy of 10.6 billion electron-volts produced copious quantities of subatomic particles called upsilons. These particles, in turn, decay to produce B and anti-B meson pairs. By using electron and positron beams of unequal energies, researchers can examine the decays of B and anti-B mesons separately.

Two groups vied to host the B Factory. SLAC worked with the Lawrence Berkeley Laboratory and the Lawrence Livermore National Laboratory on a proposal to upgrade and modify its PEP accelerator to hunt for B mesons. Cornell University, which already has an accelerator with a first-rate detector for tracking B mesons, submitted its proposal at the same time. After several years of reviews by various panels and committees, SLAC won the competition.

With the cost of its PEP upgrade put at $177 million, the SLAC proposal also calls for an additional $60 million to build a detector. A significant proportion of the funding for this detector may come from foreign sources. If construction starts this year, the SLAC B Factory should be open for research in 1998.

Aimed at the elucidation of CP violation and matter-antimatter asymmetry, the B Factory complements several other particle physics projects. At the Fermi National Accelerator Laboratory in Batavia, Ill., physicists are searching for the top quark -- the only one of the six types of quarks that remains undetected (SN: 4/24/93, p.264). At the Superconducting Super Collider (SSC), under construction in Texas, researchers will study high-energy collisions between protons and antiprotons to gain insights into the origin of mass.

Construction of the B Factory can start this year if Congress appropriates $36 million for this purpose in the fiscal 1994 budget. A conference committee of House and Senate members must also decide the fate of the SSC. Last June, the House voted by a large margin to kill the project (SN: 7/17/93, p.45). Late last month, however, the Senate decided by a 57-42 vote to continue funding the SSC.
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Title Annotation:Stanford Linear Accelerator selected as site to study B mesons
Author:Peterson, Ivars
Publication:Science News
Date:Oct 16, 1993
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