Deuteron Form Factors
The simplest bound system of neutrons and protons is the deuteron, consisting of one proton and one neutron. In the language of the theory of the strong interaction, quantum chromodynamics (QCD), it is made of six valence quarks (3 up and 3 down), plus the quark-gluon sea. In the standard proton-neutron picture, the deuteron's shape is largely determined by the exchange of a pion, which leads to strong, noncentral "tensor" interactions.
Elastic electron scattering at high momentum transfer probes the short distance structure of the deuteron's interior. With ordinary unpolarized scattering, its magnetic and electric distributions can be separated. Adding a polarization observable, such as the recoiling deuteron's "tensor polarization," makes it possible to further separate the electric piece into its charge and quadrupole components. The tensor moment t20 is most sensitive to the deuteron's charge distribution, while the other two moments, t21 and t22, provide important symmetry checks and information about the deuteron's magnetization. The shapes associated with these moments are shown in figure 1.
Two sets of experiments carried out at JLab in the late 1990s push measurements of the deuteron's electromagnetic distributions to the highest momentum transfer presently possible. An unpolarized scattering experiment in Hall A extracted the electric and magnetic structure functions, called A and B, respectively, and the results for A are shown in figure 2. An experiment in Hall C determined the tensor moments t20, t21 and t22. Combining these data has allowed the first determination of the deuteron's charge distribution as it crosses zero and provides very stringent tests of models of the force between two nucleons.