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New
Release -- Superconductor Week does not edit or endorse the following
news release:
Strange physics experiment is unraveling structure of proton
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University of
Illinois photo
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| Workers
install the 100,000 pound magnet for testing at the
University of Illinois prior to the G-Zero
experiment at Thomas Jefferson National Accelerator
Facility in Newport News, Va. |
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June 17 -- An international
team of nuclear physicists has determined that particles called strange quarks
do, indeed, contribute to the ordinary properties of the proton.
Quarks are subatomic particles that form the building blocks of atoms. How
quarks assemble into protons and neutrons, and what holds them together, is not
clearly understood. New experimental results are providing part of the answer.
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”).
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.”
The results are helping scientists better understand how one of the pieces of
the Standard Model is put together. The Standard Model unifies three forces:
electromagnetism, the weak nuclear interaction and the strong nuclear
interaction.
“The G-Zero experiment tells us more about the strong interaction – how protons
and neutrons are held together,” Beck said. “However, we still have much to
learn.”
The G-Zero experimental program is funded by the National Science Foundation,
the U.S. Department of Energy, the French National Center for Scientific
Research (CNRS) and the Natural Sciences and Engineering Research Council (NSERC)
in Canada. |
