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SLAC's Asymmetric B-Factory passes major milestone

In a cavernous room dug into a SLAC hillside, a scientific first occurred on July 23. Scientists brought together two beams ­ one made of electrons and the other of positrons ­ whirling 130,000 times per second in opposite directions through mile-long underground rings. As particles in the two beams collided, they produced microscopically small but blindingly powerful explosions that reproduce the conditions that prevailed during the earliest days of the universe.

While this was not the first time that scientists have caused high-energy beams of electrons and positrons to collide, it was the first time that the collisions occurred between beams of unequal energies. This "asymmetry" is the key to the $177-million B-Factory, which 300 physicists, technicians and workers from the Stanford Linear Accelerator Center, Lawrence Berkeley National Laboratory and Lawrence Livermore National Laboratory have labored for more than four and a half years to construct.

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"This is a truly impressive accomplishment so early in the commissioning process," said Under Secretary of Energy Ernest Moniz. "The B-Factory will help us examine one of nature's great secrets ­ why the universe has such a preponderance of matter over antimatter."

As its name suggests, the Asymmetric B-Factory has been specifically designed to produce large quantities of particles called B mesons. When electron and positron beams of the same energy collide, the subatomic particles that are created fly off in every direction from a single point. This makes it difficult to reconstruct the sequence of events that take place. Putting more energy into the electron beam than the positron beam imparts a net motion to the by-products of the resulting collisions. So events that take place at infinitesimally different times also take place at measurably different locations.

The scientists expect the information to offer new insight into the differences between normal matter and antimatter. Antimatter is the mirror image of normal matter, but with the opposite electrical charge. For example, an electron has a negative charge. Its antiparticle, the positron, has the same weight and size, but carries a positive charge. Otherwise, matter and antimatter act almost identically. Since antimatter was discovered, scientists have wondered why there is virtually no antimatter in the visible universe. The B mesons are involved in one of the few interactions where matter and antimatter behave slightly differently, so the researchers hope that measuring the difference in the lifetimes of the B mesons and their antiparticles, which are created from electron/positron collisions, will help clear up this mystery.

The major arteries of the B-Factory are the two rings. They are a pair of pipes, each about 6 inches in diameter, that have had most of the air pumped out of them. They run side-by-side through an underground tunnel about a mile in circumference. The low energy ring, which contains positrons, is elevated on 6-foot steel supports so it runs near the top of the tunnel. The high-energy ring, with its heavier magnets and fittings, runs at knee height.

The two rings consist of 1,500 individual chambers, most with their own set of vacuum pumps. Each chamber must first be "baked out" to remove volatile residues. As the air is pumped out of each new chamber, the wall separating it from its predecessor is carefully removed.

At each place where a beam turns, the piping is surrounding by heavy magnets. These generate precisely shaped magnetic fields within the pipe that deflect the particle beam while keeping it focused. Devices called klystron tubes, which were invented at Stanford, fill the rings with microwave energy that accelerates bunches of particles about the size of rice grains to velocities approaching the speed of light. Each time the path of these particles curves, they emit powerful X-rays. Special cooling jackets absorb this radiation and carry it off as heat.

The "business end" of the B-Factory is a portion of the rings called the interaction section. Here the top, low-energy ring swoops down to meet the bottom, high-energy ring and the two merge into a single ring for the space of a few meters. This is the place where the two beams come into contact.

"Getting this collider to work is something like getting a mile-long line of high-performance race cars all running at the same time," says John Seeman, head of the B-Factory commissioning team that has the year-long task of working out the bugs in the new machine and getting it to run properly.

The commissioning team had been putting in 12- to 16-hour days trying to achieve this latest milestone in time for the International Conference in High Energy Physics held in Vancouver,B.C., the week of July 21. SLAC Director Burt Richter "was trying very hard not to put pressure on us, but he kept giving us his fax and phone number in Vancouver," said Jonathan Dorfan, who is in charge of the B-Factory project.

At noon on July 23, the scientists who crowded into the B-Factory control room saw the first evidence that they had successfully gotten both beams running and brought them together to produce the first electron/positron collisions.

The significant readings took place during a monthly teleconference with Department of Energy officials in Washington, D.C., so they were among the first to get the good news. After the word spread at SLAC, more than 60 scientists, engineers and technicians crowded into the windowless room to toast the achievement with non-alcoholic champagne.

But the scientists are quick to point out that they have a lot of work yet to do before the B-Factory is fully operational.

On July 23, the low-energy beam was persisting for only a few minutes. Over the weekend the researchers realized that they did not have an adequate vacuum in the ring. Ultimately, the physicists expect beams to last for four hours. Right now, the scientists are injecting only one bunch of particles in each ring. When the machine is fully operational, each of the rings will be filled with 1,600 bunches of particles ­ about one bunch per yard.

If all continues according to schedule, then in January the length of pipe bridging the interaction section will be removed and a massive 1,000-ton particle detector known as BaBar will be trundled into position centered on the interaction region. The detector was built by an international collaboration of more than 500 physicists and engineers in parallel with the construction of the rings.

Once the detector is in place the scientists will test how well the rings and detector work together. If this testing goes smoothly, then the machine should begin its first scientific research in the spring of 1999.

The real meaning of the current milestone, say both Dorfan and Seeman, is that the collider construction is both on time and on budget. Following the superconducting supercollider (SSC) project, which was repeatedly behind schedule and over budget until Congress canceled it, the two scientists agree that it is extremely important to demonstrate that the high energy physics community can build these extremely expensive scientific instruments without the problems that plagued the SSC.


By David F. Salisbury

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