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Technologists frequently exploit the magnetism of the proton¿which Otto Stern discovered to be some three times larger than predicted by quantum mechanics in 1933. A prime example is the magnetic resonance imaging (MRI) machine, widely used in biology and medicine. But physicists still don't understand what exactly generates the particle's magnetism. The chief problem is that they can't just take a proton apart to study how its constituent parts¿the quarks¿actually contribute to its properties. But a new set of experiments reported in the December 15th issue of Science sheds some light on the role that strange quarks in particular play in the proton's magnetic pull¿and, in fact, they suggest that it's a much smaller role that previously thought.
In the 1970s, researchers investigated "strangeness" by scattering beams of particles called pions off protons and neutrons, collectively referred to as nucleons. A second line of attack, scattering with muons, followed in the 1980s. In the new study, though, a team of physicists from nine different universities tried a third approach: electron scattering that takes advantage of the fact that so-called weak forces violate mirror symmetry, or parity. "The parity-violating nature of the weak force provides a powerful tool to unravel the internal structure of the proton," says Doug Beck of the University of Illinois at Urbana-Champaign. "The basic idea is to study the preference for the proton's interaction with electrons spinning counterclockwise over those spinning clockwise."
The group first sent an intense beam of electrons scattering off a liquid hydrogen target, which suggested a potentially large contribution from strange quarks to the proton's magnetism. But in a second test, they scattered electrons off deuterium and got a different reading. "The new results imply that less than 6 percent of the proton's magnetic moment arises from the strange quark," Beck says. The physicists ascribe the difference to an effect long predicted but never measured¿the so-called anapole moment of the proton. They plan to continue their work using a special toroidal superconducting magnet, which is now undergoing testing. "The new magnet should allow measurement of the anapole moment with much greater precision over a wide range of momentum transfers," Beck comments. "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."
