Spinning the nucleon into sharper focus
DOI: 10.1063/1.2408525
In the simplest picture, a proton or neutron is made up of three “valence” quarks. A more complete picture also includes not only a sea of quark–antiquark pairs that pop in and out of the vacuum but also gluons, which hold the quarks together. Working at the Thomas Jefferson National Accelerator Facility in Virginia, a multinational research team has precisely measured the distribution of spin for a neutron’s valence quarks. Firing a 5.7-GeV polarized electron beam at a polarized helium-3 target, the researchers could focus on the valence quarks by choosing an excitation region where electron interactions with gluons and sea quarks are screened out. The physicists then combined their new neutron data with existing proton data and concluded that 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. The results agree well with predictions from the relativistic constituent quark model, which considers the quarks’ orbital angular momenta within a nucleon. However, they disagree with predictions from a commonly used approximation of perturbative quantum chromodynamics that does not account for the quarks’ orbital angular momenta. (
