Bringing the Nucleon into Sharper Focus

Working at the Thomas Jefferson National Accelerator Facility in Virginia, a multinational research team has determined how quarks in a proton orient their "spins," which, roughly speaking, can be visualized as tiny bar magnets that point in a certain direction and have a certain strength. Information about a quark's spin can provide new details of how the tiny particles arrange themselves inside a nucleon (proton or neutron).

In high-school physics classes, students are taught that a proton or neutron simply consists of three quarks, which specialists call "valence quarks." A more complete picture includes these three valence quarks, plus a sea of quark-antiquark pairs that pop in and out of empty space (the vacuum), as well as particles called gluons which hold the quarks together.

Now, for the first time, researchers have precisely measured the distribution of spin for a neutron's valence quarks. Strikingly, their results reveal the importance of once-neglected orbital motions of quarks inside the nucleon.

Aiming an electron beam at a helium-3 target in JLab's Hall A, researchers (led by Jian-Ping Chen, jpchen@jlab.org and Zein-Eddine Meziani, meziani@temple.edu) selected a 5.7 GeV beam energy so that the electrons interacted mainly with the neutron's valence quarks and not its sea quarks and gluons.

Interestingly, the researchers applied their new neutron data, along with existing proton data, to find out more about the proton. Their conclusions: the spins of the proton's two valence up quarks are aligned parallel to the overall proton spin, but the same is not true for the proton's valence down quark (see image).

This result disagrees with predictions from an approximation of perturbative quantum chromodynamics (pQCD), a widely accepted theory of the strong force (which holds the nucleon together). This approximation does not account for the quarks' orbital angular momenta, which describes the orbital paths of quarks inside the nucleon.

However, the results agree well with predictions from a relativistic valence quark model, which does consider quarks' orbital angular momenta as they move inside the nucleon. (Zheng et al., Physical Review Letters, upcoming article; for more information, contact Xiaochao Zheng, Argonne, 630-252-3431, xiaochao@jlab.org)

Once omitted in simpler pictures of the nucleon, quark orbital angular momentum is also proving important for exploring questions about the shape of the proton (see for example New Scientist, May 3, 2003.)