New Results Hint at Strangely Magnetic Proton

New results from experiments performed at the DOE’s Thomas Jefferson National Accelerator Facility indicate that strange quarks may contribute to the proton’s magnetic moment, according to Krishna Kumar, a physicist at the University of Massachusetts, Amherst and member of the Hall A Proton Parity Experiment (HAPPEx). If these preliminary findings are confirmed later this year, it would mean that strange quarks in the proton’s quark-gluon sea contribute to at least one of the proton’s intrinsic properties.

The experiment measures the neutral weak force between a polarized beam of electrons and target nuclei of hydrogen and helium-4 at a length scale of around one femtometer. The electromagnetic force conserves parity, while the weak force is not, so measuring the fractional difference in the number of scattered electrons due to the beam’s changing polarization allows researchers to calculate the neutral weak force.

Physicists hope to use these measurements to learn about the strong force that binds up and down quarks into protons and neutrons, as well as the up, down and strange quark contributions to the nucleon’s charge and current distributions. That’s because the neutral weak force measurement is sensitive to the "weak" charge and current distributions inside nucleons, as opposed to the corresponding electromagnetic distributions. Thus, says Kumar, "One can infer whether s-quarks contribute to the charge and current distributions."

According to Kumar, the results indicate that the strange quark contribution to the nucleon’s charge and current distribution is zero within the sensitivity of each measurement. However, he added, "There seems to be a trend towards a positive value for the average contribution of strange quarks to the proton’s magnetic moment." This result will be "surprising and exciting," said Kumar, if it is confirmed with more precise measurements planned by HAPPEx later this year.

Data from several other recent experiments–including SLAC’s E158, the SAMPLE experiment at MIT-Bates, the A4 experiment at Germany’s Mainz Laboratory, and the G-Zero experiment at JLab–are also beginning to shed light on the weak interaction.