PEP-II Moves to New Resonance
Compelled by the abrupt change in the lab's budget, researchers have rapidly altered the B-Factory's program in order to continue doing exciting physics the remaining two months of the run. The sudden change required extra efforts during the winter shutdown to adjust the PEP-II machine to run at a different collision energy than normal.
The experiment is now running in the 3S mode, which means particles collide with a total energy of 10.35 GeV, enough to make particles called Upsilon(3S), which then decay into other particles. For most of its 8-year-lifespan, PEP-II has operated in the 4S mode, with collisions at 10.58 GeV that produce Upsilon(4S) particles. This rather small energy difference makes a big difference in how the machine needs to be set up and allows searches for new physics and new particles that would have been difficult to find at 4S.
This is almost new territory for PEP-II, which has been run at 3S before—during a few shifts five years ago—and had been slated to operate at the 3S for a few weeks this summer to pick up this interesting data.
To adjust the machine, the Linac Operations Group switched off several klystrons in the linac, so the electron beam now gets accelerated to only 8.6 GeV, instead of 9 GeV. PEP-II crews, led by Michael Sullivan of the Accelerator Systems Department, proportionately scaled down the strength of PEP-II magnets that steer and focus the high-energy electron beam. The resonance, or ideal energy, at which Upsilon(3S) particles can be made is very narrow—a bell curve with a narrow top—so researchers worked carefully to match the energy of the collisions with the peak of the 3S resonance where the most particles are produced.
Because the 3S resonance is narrower than the 4S one, PEP-II operators have to keep a closer eye on the collision energy. "If the machine moves off the resonance by even 0.0001 GeV, we have to re-center it," said John Seeman, head of the Accelerator Systems Division. "But operating at 3S is well within the parameters of the machine."
Lowering the energy of the electron beam has a fringe benefit: the beam size is now smaller. Packing the same number of electrons into a smaller bunch increases the luminosity, or number of collisions per second—in this case by about 15 percent.
Heather Rock Woods, SLAC Today, January 16, 2008
Above image: The two beam pipes of the PEP-II storage ring—the upper pipe carries positrons, the lower pipe carries electrons. (Image courtesy of Peter Ginter.)