December 12, 2003  


PEP-II on Track to Nearly Double BABAR Data Sample

By Kate Metropolis

SLAC set an ambitious goal for run 4 of the PEP-II accelerator: 100 inverse femtobarns. An inverse femtobarn measures the intensity of the beams, how frequently they cross, and how long the run lasts. If this goal is reached, the data sample of the BABAR collaboration from the first three runs would be nearly doubled by July 2004. Three months into this run, the accelerator is performing beautifully. Recent modifications of PEP-II’s hardware and operations have allowed it to maintain more intense beams—and the future looks bright.

Accelerator operators (l to r) John Amann, Stephen Weathersby and Tom Sommer are three of the hundreds of people intent on delivering one of the greatest data samples in the history of particle physics. Physicists, engineers, technicians and machinists—some on-call around the clock—all contribute to the effort. (Photo by Diana Rogers)

"It’s no exaggeration to say that this is one of the most complicated machines on Earth," says Roger Erickson (AD). The accelerator’s basic mission is to deliver to the BABAR detector as many collisions as possible between a positron and an electron, whose energies are set to produce a particular pair of particles: a B-zero and an anti B-zero meson. Particle physicists call this pair ‘B, B-bar’—hence the origin of the name BABAR. The B mesons quickly decay into other particles, which fuel the research of some 150 different analysis projects in the BABAR collaboration.

The BABAR detector recorded some 125 million B, B-bar pairs from October 1999 to July 2003, but physicists are eager for more. "PEP-II is providing us with one of the great data samples in the history of particle physics," says BABAR physics analysis coordinator Jeff Richman.

"With our current data, we’ve already found new ways to explore the difference in behavior between matter and antimatter, discovered an intriguing particle that casts new light on the strong force between quarks, and found many new decay processes."

"But," he added, "we can’t rest on our laurels. If we stopped getting more data now, it would be like sealing off the entrance to a mine that we knew still had a lot more diamonds. And there could be even more surprises."

New equipment is part of the approach. An eighth radio frequency (RF) cavity was added to the accelerator, allowing more particles to be stored in the ring. This device boosts a bunch of electrons along on electromagnetic waves the way an ocean wave boosts surfers. Another improvement was to solve the problem of unwanted electrons in the positron ring. These electrons, kicked loose from the beam pipe by synchrotron light radiated by the orbiting positrons, diffuse the tightly packed positron beam, which lowers the chances of collisions with the electron beam in the detector. Technicians spent grueling weeks in a hot tunnel, winding narrow wire tape around every accessible part of the beam pipe in the positron ring. The windings created a solenoid magnet that traps the slower electrons and keeps them out of the positrons’ way.

Maintenance is another important ingredient. Over the summer, a vacuum leak in the interaction region was quickly repaired by the Mechanical Fabrication Department, and the Accelerator Maintenance RF group overhauled the entire RF system. Accelerator physicist Mike Sullivan (AD) gives kudos to both groups for ensuring that starting the accelerator back up was smooth and straightforward.

New ways of operating the accelerator have also started to pay off . Just as people have to sit in a seat to be whirled around an amusement park ride, particle bunches must sit in just the right spot, called a ‘bucket,’ on an electromagnetic wave to be accelerated around the rings. Last year, PEP-II operated with two empty buckets following each filled bucket. This fall, the pattern was changed: strings of buckets in which every other one is filled alternate with shorter strings of empty buckets.

Each change to the spacing between bunches affects the beams’ behavior. "Now," says Erickson, "we’re learning the physics of PEP-II when filled buckets sit closer to each other, and this new pattern has opened up empty slots to which we can eventually add more particles."

A new approach to keeping the rings full was adopted at the beginning of December. As the beams collide, their intensity gradually declines. Previously, it was necessary to ‘top off’ the beams by injecting new particles every 50 minutes or so. During the five or ten minutes injection takes, the detector had to be shut off to avoid the risk of radiation damage. A new trickle injection scheme in the positron ring adds tiny pulses of particles as soon as the buckets begin to be depleted, maintaining the beam at full brightness around the clock. This approach has a double data payoff: the collision rate does not fall off and, since the detector is desensitized for much less time, it can record up to 20 percent more events.

The ambitious plan for run 4 doesn’t make department head John Seeman (AD) flinch. He simply says, "That’s why we’re here."


The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Thursday December 11, 2003 by Emily Ball