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Particle beams clash for first time at new collider

A sophisticated new "particle smasher" at Stanford University built to explore the difference between matter and antimatter sprang to life this week with the successful achievement of its first collisions.

Physicists at the Stanford Linear Accelerator Center (SLAC) circulated beams of electrons and and their antimatter opposites, known as positrons, simultaneously in two evacuated rings, each more than a mile around. When they brought the narrow beams together at a single crossing point, they observed the deflection and disruption of one beam by the other ­ a sure sign that head-on collisions had occurred.

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Funded by $177 million from the U.S. Department of Energy, the Asymmetric B Factory is a joint construction project of SLAC, Lawrence Berkeley National Laboratory (LBNL), and Lawrence Livermore National Laboratory (LLNL). The dual-ring machine is an extensive upgrade of an existing electron-positron collider at SLAC. Construction began in 1994 and was completed in early July, on budget and on schedule.

"I am very pleased that we have achieved collisions so soon after finishing construction," said SLAC physicist Jonathan Dorfan, the project leader. "We look forward with great anticipation to completing the commissioning process and beginning the physics program next year."

The collider is now in the midst of a long tuning process, called commissioning, that will continue into the spring of 1999, when the machine will begin operations for physics research. In January a 1,000-ton particle detector known as BaBar will be moved into position at the point where the two beams intersect.

Built by a large international collaboration of more than 500 physicists and engineers, the massive detector is designed to search through the debris of electron-positron collisions for evidence of short-lived subatomic particles known as B mesons. By comparing the production and disintegration of these particles with those of their antiparticles, physicists hope to learn more about the differences between matter and antimatter that led to the universe being composed almost entirely of matter.

Under Secretary of Energy Ernest Moniz congratulated the three laboratories involved and especially the B Factory team, saying, "This is a truly impressive accomplishment so early in the commissioning process. The B Factory will help us examine one of Nature's great secrets ­ why the universe has such a preponderance of matter over antimatter."

The Asymmetric B Factory is the world's first particle collider in which the electrons and positrons meet at unequal energies: electrons have almost three times the energy of positrons. Because of this difference, plus the need to circulate high currents in order to produce millions of B mesons, physicists have designed a machine in which the two different kinds of particles travel in two separate rings.

A complex array of magnets before and after the crossover point brings the beams together and then separates them after they clash. These magnets also focus the beams down to small dimensions in order to enhance the chances of obtaining electron-positron collisions.

Commissioning of the B Factory has gone smoothly, with few surprises, which SLAC Director Burton Richter called "a testament to the expertise of the accelerator physicists from the three laboratories who designed and built this machine."

The B Factory will resume operations in October with attempts to boost the collision rate.


By David F. Salisbury

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