Where does the proton get its spin? This question has puzzled physicists ever since experiments in the 1980s revealed that a proton’s constituent quarks—the most fundamental building blocks of atomic nuclei—account for only about one-third of a proton’s spin. Collisions of spin-polarized protons at the Relativistic Heavy Ion Collider (RHIC), a U.S. Department of Energy Office of Science user facility for nuclear physics research at DOE’s Brookhaven National Laboratory, are helping to solve this mystery.
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Source: Phys.org