Ever since Otto Stern surprised his colleagues in 1933 by announcing that the proton’s magnetic moment was some three times as large as expected, physicists have puzzled over the origin of this effect. 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’s magnetism as seen by the weak, rather than the electromagnetic, interaction.
Although the weak interaction violates parity (left/right symmetry), it still tries to mimic electromagnetism, and this introduces a magnetic-like term, which was called “weak magnetism” by Gell Mann in 1958.
The new measurement leads to the first direct information on how different quark “flavours” in the proton generate the magnetic moment. Because the electromagnetic and weak interactions are precisely related in the Standard Model, the new result can be combined with the proton’s ordinary magnetic moment (and that of the neutron, the proton’s iso-spin partner) to uncover the magnetic contributions of the separate quarks.
The experiment is an analogue of the classic electron-scattering experiments of Robert Hofstadter and his collaborators at Stanford in the 1950s. In the SAMPLE experiment, the electrons are polarized so that their spins are aligned either parallel or antiparallel to the beam direction. Scattering experiments with these two types of beam are sensitive to the mirror-symmetry (parity) violating nature of the weak interaction. However, the relative differences are only a few parts per million, presenting a significant experimental challenge.
An intense pulsed beam of polarized electrons from the Bates accelerator hits a liquid-hydrogen target. The backward-scattered electrons are detected with a large solid-angle air Cherenkov detector.
It is the strange quark contribution to the magnetic moment that is of the greatest interest, because such effects must come from the proton’s “sea” of virtual quark-antiquark pairs.
In the first SAMPLE experiment, the parity-violating asymmetry of the proton was measured. Using a theoretical estimate for the contribution of the weak axial vector current, the portion of the magnetic moment due to strange quarks comes out to be significantly positive.
To check the axial current contribution, a second measurement was made last summer using a deuterium target, where the strange quark effects from the proton and neutron are expected largely to cancel. The analysis will reveal the strange quark contribution to the proton magnetic moment.
These experiments, plus new measurements from the Jefferson Lab, mark the beginning of a programme to determine the contributions of strange quarks to the proton’s inner distributions of charge and magnetization.
SAMPLE is a collaboration between Caltech, Illinois, Louisiana Tech, Maryland, MIT, William and Mary, and Virginia Tech.
