PEP-II's luminous life
By Mason Inman
Just as proud parents mark their children's height on the kitchen wall,
on April 12 SLAC staff marked a major achievement in the life of PEP-II.
(Photo by Peter Ginter)
"We've delivered a total of 200 inverse femtobarns to
detector since the start of the project," said Michael Sullivan
(PEP-II). An inverse femtobarn is a measure of the number of particle
collisions in a period of time. Since July 1999, PEP-II has been
colliding electrons and positions to produce B and anti-B mesons as
fodder for the BABAR
More collisions mean more data for experimentalists to analyze. "They're
working hard to keep up," Sullivan said.
PEP-II is now smashing particles like never before. The whole system has
been continually tuned and improved over its five-year lifespan, and
it's currently producing collisions at roughly three times the rate it
was in its early days.
PEP-II took about 27 months to deliver its first 100 inverse femtobarns,
Sullivan said. Because of all the improvements, the current run, which
ends in July, should deliver a total of 100 inverse femtobarns in only
nine months. This would meet a goal the team set for themselves at the
beginning of the run, and bring the lifetime total to roughly 250
More collisions isn't the only benefit of making PEP-II run more
smoothly. "When the machine is well-tuned, the backgrounds tend to
remain low," Sullivan said. When the backgrounds are lower, then the
data pouring out of PEP-II is less noisy, so experimentalists have to
sift through less data to find the interesting physics.
Three recent improvements to PEP-II have increased its luminosity, a
measure of how well the electron and positron beams are colliding, to a
new record for the machine. The electron storage ring was switched over
to trickle charge mode in March, in which more electrons are injected
continuously into the ring, rather than in spurts throughout the day.
Keeping the number of electrons high all the time increases the number
of collisions that are possible.
Also, the number of bunches of electrons and positrons traveling around
the rings was edged up to 1,556, a 15 percent increase, which brings the
machine closer to its limit of 1,700 bunches. Yet another improvement
was in squeezing down the beams, making them denser and making
electron-positron collisions more likely. All these improvements have
brought PEP-II to a peak luminosity of 8.34 x 1033 cm-2s-1, approaching
triple the luminosity for which the machine was designed.