Quark Spin in a Proton

In experiments at Jefferson National Accelerator Facility in Virginia, a multinational research team has found that the spins of the proton's two up quarks (u) are aligned parallel to the overall spin of the proton, but the same is not true for the proton's down quark (d).

In order to make the experimental data on quark spin agree with theory, researchers had to take into account the once-neglected orbital motions of quarks inside the proton. Therefore, these experiments provide many new details on the behavior of quarks in the proton. (Figure courtesy Jefferson Lab.)

Reported by: Zheng et al., Physical Review Letters, upcoming issue

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