Strange Magnetism and the Proton

In 1933, the German physicist Otto Stern discovered that the magnetism of the proton was anomalously large, a factor of three larger than expected from the basic theory of quantum mechanics, a discovery that offered the first glimpse of the internal structure of the constituents of the atomic nucleus and a tantalizing hint at the existence of quarks. Widespread applications of the proton's magnetic properties, such as the magnetic resonance imaging (MRI) techniques used in biology and medicine, have been developed despite a lack of fundamental understanding of the basic dynamics that generates this magnetism. After the key discovery of internal structure in the proton in a high-energy electron scattering experiment at the Stanford Linear Accelerator Center in the late 1960s, the theory of quantum chromodynamics (QCD), which describes the interaction between quarks and the gluons that bind the quarks into the atomic nuclei observed in the periodic table, was developed. Despite almost 30 years of intense theoretical effort, QCD has been unable to produce numerical predictions for the basic properties of nucleons, such as their degree of magnetism.

A group of scientists led by Professor Douglas Beck and co-leaders Bob McKeown, (Caltech), Betsy Beise (Maryland), and Mark Pitt (VPI) has studied the internal structure of the proton using the violation of mirror symmetry in the weak force in a series of experiments—dubbed SAMPLE—at the MIT/Bates linear accelerator. In an article in this week's (15 December 2000) Science magazine, experimental results are reported that address the role of strange quarks in generating nuclear magnetism. The measurement reported provides an unambiguous constraint on strange quark contributions to the proton's magnetic moment through the electron-proton weak interaction. Evidence is also shown for the existence of a parity-violating electromagnetic effect known as the anapole moment of the proton. The proton's anapole moment is not yet well understood theoretically, but it could have important implications for precision weak interaction studies.

The figure shown above represents the results of a combined analysis of the data from two SAMPLE measurements. The two error bands from a hydrogen experiment and a deuterium experiment are indicated. The inner hatched region includes the statistical error and the outer represents the systematic uncertainty added in quadrature. The ellipse represents the allowed region for both form factors at the 1σ level. Also plotted (vertical bar) is the present best calculation of the isovector axial e-N form factor G_A^e (T = 1), obtained by using the anapole form factor and radiative corrections. This calculation suggests a large contribution from these effects; the experiment confirms the large contribution and suggests that it may be larger still.

Further information about the SAMPLE experiments is available from Professor Beck.

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