"And they're off!" might have been the
battle cry at 4 a.m. on July 16, when the Low Energy Ring (LER) of the B-factory stored
its first beam of positrons (positively-charged electrons). With the B-factory's High
Energy Ring (HER) having begun storing beams of electrons a short while earlier,
construction of the $230 million particle collider -- a collaboration between
Berkeley Lab, the Stanford Linear Accelerator Center (SLAC), and Lawrence Livermore
National Laboratory (LLNL) -- appears on schedule for its targeted completion
date of October of this year.
The historic event took place following five "hectic" days of commissioning.
Success was achieved when the LER stored a one milliampere beam of positrons for
approximately 30 seconds while meeting all of its design parameters.
The PEP-II facility, or "B-factory," is a
collaborative project between Berkeley Lab, SLAC and LLNL. Built to produce
"copious" quantities of B mesons, the B-factory could help show why the process
of creation favored matter over antimatter.
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"The LER is one of the most challenging storage rings ever designed," says
Michael Zisman, an Accelerator and Fusion Research Division (AFRD) physicist in charge of
commissioning the LER. "Congratulations to all of the hard-working LER commissioning
team for this extraordinary effort."
"B-factory" is the popular name given to the conversion of SLAC's PEP
collider (Positron-Electron Project) into PEP-II, an "asymmetric" collider --
one in which the two colliding beams of particles are not of equal energies. For the
B-factory, the asymmetric rings are the HER, which will store electrons at an energy of 9
GeV (billion electron volts), and the LER, which will store positrons at 3.1 GeV. The
rings, both of which measure nearly 2.2 kilometers in circumference (1.36 miles), are
stacked together in the same tunnel with the LER, positioned immediately above the HER.
Primary responsibility for the design, construction and commissioning of the LER belongs
to Berkeley Lab. The HER is an upgrade of the old PEP ring.
The purpose of the B-factory is to produce copious quantities (with factory-like
reliability) of B mesons, particles containing a "bottom" quark, the fifth of
the six quarks believed to be fundamental constituents of matter. Measuring the lifetimes
of B mesons and their antiparticle counterparts, B-bars, offers scientists their best
opportunity to study differences between matter and antimatter, particularly the
phenomenon known as CP violation (charge-conjugation/parity).
This phenomenon is widely believed to be responsible for the fact that during the first
split seconds of the Big Bang, the process of creation favored matter over antimatter.
The use of an asymmetric collider to produce B particles was first proposed in 1987 by
Pier Oddone, then head of Berkeley Lab's Physics Division and now one of the Lab's deputy
directors. The idea is that an electron-positron collision yields a particle known as the
upsilon 4S, which immediately decays into a B meson and a B-bar. These B particles in turn
decay into a host of other charged and neutral particles whose detection can provide a
wealth of information.
In all previous colliders, where the energies of the two beams were equal, the newly
created B particles would have remained nearly stationary, making it difficult to study
their decay products. In an asymmetric collider, however, the B particles are carried down
the beamline in the direction of the more energetic beam. This forward motion with respect
to a laboratory frame of reference causes their decay products to separate in space and
time.
"This separation permits the reconstruction of the individual B mesons and the
study of the time evolution of their decays," Zisman says.
The decay of a B meson is such a rare event that nearly a hundred million B/B-bar pairs
must be produced in order for scientists to record any significant data. This requires a
collider of unprecedented "luminosity" -- a measurement of the rate of particle
collisions over space and time. The B-factory's design calls for a luminosity of 3 x 1033
(3 followed by 33 zeros) collisions per square centimeter per second, which is at least 10
times better than the highest luminosity achieved with the best machines in the world
today.
"The study of B meson decays will be one of the key elements of worldwide high
energy physics investigations for many years to come," Zisman says.
Components of the asymmetric beamlines for the
B-factory being built at SLAC include the upper raft, which holds the magnets for the Low
Energy Ring, and the lower raft, a refurbished dipole for the High Energy Ring.
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