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The experiment, called G-Zero, was performed at Thomas Jefferson
National Accelerator Facility in Newport News, Va. Designed to probe
proton structure, specifically the contribution of strange quarks,
the experiment has involved an international group of 108 scientists
from 19 institutions. Steve Williamson, a physicist at the University
of Illinois at Urbana-Champaign, is the experiment coordinator.
"The G-Zero experiment provided a much broader view of the small-
scale structure of the proton," said Doug Beck, a physicist at
Illinois and spokesman for the experiment. "While our results agree
with hints from previous experiments, the new findings are
significantly more extensive and provide a much clearer picture."
Beck will present the experimental results at a seminar at the
Jefferson facility Friday morning. Also on Friday, the researchers
will submit a paper describing the results to the journal Physical
Review Letters. The paper will be posted on the physics archive
(under "nuclear experiment") at www.arxiv.org.
The centerpiece of the G-Zero experiment is a doughnut-shaped
superconducting magnet 14 feet in diameter that was designed and
tested by physicists at Illinois including Ron Laszewski, now
retired. The 100,000-pound magnet took three years to build.
In the experiment, an intense beam of polarized electrons was
scattered off liquid hydrogen targets located in the magnet's core.
Detectors, mounted around the perimeter of the magnet, recorded the
number and position of the scattered particles. The researchers then
used mathematical models to retrace the particles' paths to determine
their momenta.
"There is a lot of energy inside a proton," Beck said. "Some of that
energy can change back and forth into particles called strange
quarks." Unlike the three quarks (two "up" and one "down") that are
always present in a proton, strange quarks can pop in and out of
existence.
"Because of the equivalence of mass and energy, the energy fields in
the proton can sometimes manifest themselves as these 'part-time'
quarks," Beck said. "This is the first time we observed strange
quarks in this context, and it is the first time we measured how
often this energy manifested itself as particles under normal
circumstances."
