A major upgrade of the Beijing
Electron–Positron Collider will give it the world's highest luminosity in the collision
energy range 2 to 4.2 GeV, a physics-rich niche that, China's high-energy physicists hope,
will attract international collaborators. The $77 million BEPC II is scheduled
to start up in September 2007.
Not much of the original
BEPC, which went on line in 1988, is to be found in the revamped facility. The upgrade uses the same
tunnel for collisions and some old magnets and existing infrastructure, but it incorporates superconducting
magnets and has separate storage rings, each nearly 240 meters in circumference, for the
electrons and positrons. And whereas the original BEPC storage ring held single bunches of electrons
and positrons, the new rings will each hold 93 bunches of particles, with a bunch spacing
of 8 nanoseconds. Chen Hesheng, director of the Chinese Academy of Sciences' Institute
of High Energy Physics, which runs the collider, says that the multiple bunches, plus tighter focusing
of the beams, increases the luminosity by a factor of 100.
The enhanced luminosity
will also be more than 10 times higher than the present best for the same energy regime at Cornell
University's CESR-c. The BEPC II, like its predecessor, creates electron–positron
collisions of a few GeV, in the so-called tau–charm region, which produces tau leptons—the
heaviest of the leptons—and a variety of charmed mesons and other mesons that carry both a
charm and an anticharm quark (charmonium). Due to space and cost constraints, the maximum collision
energy of the new machine will fall somewhat short of the original BEPC's 5 GeV.
In addition to the high-energy
physics experiments, parasitic synchrotron light beams will run at the BEPC II starting
this fall. The 2.5-GeV synchrotron will provide hard x rays, as did the earlier version of
the BEPC. A more sophisticated synchrotron light source is under construction in Shanghai (see
the story on page 23).
Increased statistics
Perhaps the best-known advance made
with the BEPC was the most precise measurement to date of the tau mass, in 1996. Among the BEPC's other
credits, says Chen, are a possible sighting of a new particle and measuring the R-value,
the ratio of the cross section for hadron production to the cross section for producing muon pairs.
That ratio determines vacuum polarization and the effective value of the fine structure constant
at the mass of the Z particle and impacts standard model predictions of the Higgs mass.
In addition, says Fred
Harris, who leads a group from the University of Hawaii that works on the Beijing Spectrometer,
the sole detector on the collider, "We have gathered 58 million J/Ψ
events and 14 million Ψ'
events. There is a tremendous amount of physics one can do with such large samples of charmonium
events. We can analyze lots of different decay modes." The Hawaii group is contributing a time-of-flight
calibration system to the new incarnation of the BEPC II's detector, known as BES 3.
In going to higher luminosity
with the BEPC II, physicists hope the greater statistics will lead to a better understanding
of previously spied reactions and reveal new particles. Besides going after better measurements
of tau properties and decay modes, "One of the hot topics is D mixing, where a D [meson] can turn into
an anti-D," says Harris. "If you actually see such a thing, it's an indication of new physics." Other
key areas of research with the BEPC II will be looking at the decay of charmed mesons and "seeking
states made of gluons or gluons and quarks," says Carnegie Mellon University's Fred Gilman. "Ever
since QCD [quantum chromodynamics] was invented, theory has said that gluons should form states
by themselves—glueballs—but we haven't found them yet."
Expanding globally
The scientific promise of the BEPC II
has veteran users hoping to increase the number of participating foreign physicists. And they're
betting that the timing of their machine's startup will be a plus. In the global particle-physics
community, the Large Hadron Collider (LHC) will be in the spotlight once it goes on line at CERN next
year. But other accelerator experiments, including those at SLAC's BaBar, Fermilab's Tevatron,
and Cornell's CESR-c, are winding down.
"It's a very opportune
time," says SLAC's Wolfgang Panofsky, a longtime adviser to the Beijing collider, "because there
is expected to be quite a gap, in particular for young people, in the time frame between 2008, when
most American machines will shut down, and the startup of the International Linear Collider [ILC],
at the earliest maybe in 2017. So during this time, opportunities for the younger generation are
pretty scarce." Collaborations on the LHC may have 2000 members or so, adds Harris. "They're huge
and socially potentially harder to work with. BES 3 is a relatively small collaboration."
Over time, the Beijing
Spectrometer team has included scientists from the US, South Korea, Japan, Russia, and Europe,
but they made up a small fraction of the collaboration, which remains predominantly Chinese. Joint
work in high-energy physics, says Panofsky, "is possibly the oldest collaboration between China
and the US in basic science." It goes back, he recalls, to the US–China science and technology
cooperation agreement signed in 1979 by President Jimmy Carter and Premier Deng Xiaoping, and
"has been enduring and quite productive, and established lots of personal linkages."
At a June workshop in Beijing,
scientists and funding-agency representatives gathered to explore expanding US–China
collaborations through the BEPC II, a proposed neutrino experiment at Daya Bay in southern
China, and astrophysics and astronomy projects. Gilman, who with Chen organized the workshop,
says the turnout for the BEPC II was small, "but people from several [US] universities are
interested, and we're hoping that the collaboration will start to gel." Discussions with NSF and
the Department of Energy are starting, he adds. "We need to get their interest, to see on what level
they'll want to support this collaboration." Says Chen, "We should work together to share technology,
share the cost, share the manpower, share the experience. That is the intrinsic nature of a particle-physics
experiment."
High-energy physicists
widely see increased international collaboration on the BEPC II as a natural step toward
China's playing a significant role in the ILC. "We have joined the discussion," says Chen. "But
I can't see any serious commitment to host the linear collider. It will take time. But certainly
the Chinese particle-physics community is interested in the linear collider, and we will actively
join the collaboration."
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