Feb. 4, 2000 CHAMPAIGN, Ill. -- Since Otto Stern first surprised his colleagues in 1933 by announcing that the proton magnetic moment was three times larger than expected, physicists have puzzled over the origin of the difference. During the past two summers the SAMPLE experiment at the MIT-Bates Linear Accelerator Center has shed new light on this question by measuring the proton magnetic moment as seen by the weak interaction, rather than the electromagnetic interaction.
"The new measurements provide information about how the different flavors of quarks in the proton generate the magnetic moment," said Doug Beck, a physics professor at the University of Illinois and a collaborator on SAMPLE. "Because the electromagnetic and weak interactions are very precisely related in the Standard Model of particle physics, the new experimental result can be combined with the ordinary proton magnetic moment to uncover the contributions of the up, down and strange quarks."
In the SAMPLE experiment, an intense beam of polarized electrons is scattered off a liquid hydrogen target. The backward-scattered electrons are detected with a Cerenkov detector. Because the counting rate must be very high to accumulate the necessary statistics for a part-per-million measurement, the detector signals are integrated rather than counted.
"The electrons are polarized so their spins are aligned either parallel or anti-parallel to the beam direction," Beck said. "Scattering experiments with these two types of beams are mirror images of each other and are therefore sensitive to the parity-violating nature of the weak interaction." It is the strange quark contribution to the magnetic moment that is of greatest interest because any such effects must come from the virtual quark-antiquark "sea" in the proton, Beck said. The effects of the sea on "large scale" proton properties such as the magnetic moment are largely unknown. In the first SAMPLE experiment, the parity-violating asymmetry of the proton was measured. "Using a theoretical estimate for the contribution of the weak interaction axial current, the portion of the magnetic moment due to strange quarks comes out to be significantly positive, contrary to most theoretical models," Beck said. The SAMPLE experimenters announced their most recent findings in the Jan. 31 issue of Physical Review Letters.
"In order to check the axial current contribution, a second measurement was made this summer using a deuterium target, where the strange quark effects from the proton and neutron are expected to largely cancel," Beck said. "Upon completion of the analysis of these data, the strange quark contribution to the proton magnetic moment should be cleanly determined."
The SAMPLE experiment is a collaboration between the U. of I., California Institute of Technology, Louisiana Tech University, University of Maryland, Massachusetts Institute of Technology, College of William and Mary, and Virginia Polytechnic Institute and State University.
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