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
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
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
"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
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."