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Strange quark contribution to proton structure yields surprising result

James E. Kloeppel, Physical Sciences Editor
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CHAMPAIGN, Ill. -- Scientists seeking to confirm earlier measurements of the strange quark's contribution to the proton's magnetic moment have found several surprises, instead.

As reported in the Dec.15 issue of the journal Science, physicists who combined results from two measurements have calculated a lower strange quark contribution, and provided evidence for the existence of a parity-violating electromagnetic effect known as the proton's anapole moment.

In experiments with the SAMPLE apparatus at the MIT/Bates Linear Accelerator Center, scientists used the weak magnetic force to uncover the contributions of the up, down and strange quarks.

"The parity-violating nature of the weak force provides a powerful tool to unravel the internal structure of the proton," said Doug Beck, a physics professor at the University of Illinois and a collaborator on SAMPLE.

"The basic idea is to study the preference for the proton's interaction with electrons spinning counterclockwise, over those spinning clockwise."

In the first SAMPLE experiment, the parity-violating asymmetry of the proton was measured by scattering an intense beam of electrons off a liquid hydrogen target. In the second experiment -- conducted during the summer of 1999 -- the hydrogen target was replaced with deuterium. By scattering electrons off both protons and neutrons, the researchers could examine - and solve - different linear combinations.

While results from the initial experiment indicated a potentially large contribution from the strange quark, the second experiment "brought that number down considerably," Beck said.

"The new results imply that less than 6 percent of the proton's magnetic moment arises from the strange quark." The difference in values between the two experiments, the scientists say, is due to the presence of a surprisingly large anapole moment of the proton -- an effect long predicted, but never measured.

"The proton's anapole moment is not yet well understood, but could have important implications for precision weak interaction studies in atomic systems such as cesium," Beck said.

The spatial dependence of the strange quark contributions will be studied in detail in an upcoming program of parity-violation measurement at the Thomas Jefferson National Accelerator Facility. For that series of experiments, a special toroidal superconducting magnet -- funded by the National Science Foundation -- has been constructed and is currently being tested at the UI.

"The new magnet should allow measurement of the anapole moment with much greater precision over a wide range of momentum transfers," Beck said. "Instead of seeing the proton's overall magnetic moment, we will be able to vary the size of our probe to study small structures within the proton."

The SAMPLE experiment is a collaboration involving the UI, Argonne National Lab, California Institute of Technology, University of Kentucky, Louisiana Tech University, University of Maryland, Massachusetts Institute of Technology, College of William and Mary, and Virginia Polytechnic Institute and State University.