New data add detail—and complexity—to an intriguing puzzle that scientists have been seeking to solve: how the building blocks that make up a proton contribute to its spin. The STAR experiment at the Relativistic Heavy Ion Collider (RHIC) provided the data to help put the pieces of the puzzle together. The new results reveal definitively for the first time that different "flavors" in the "sea" of antiquarks inside the proton contribute differently to the proton's overall spin. The flavors contribute in a way that's opposite to those flavors' relative abundance.
Spin is a fundamental property of particles, as essential to their identity as electric charge. Aligning and flipping the polarity, or direction, of proton spin is the basis for technologies like magnetic resonance imaging (MRI). However, scientists still don't know how the proton's inner building blocks (quarks, gluons, and a sea of quark-antiquark pairs that form when gluons split)—as well as these particles' motions within the proton—build up the overall spin. Solving this puzzle may help scientists understand how the complex interactions within the proton give rise to its overall structure, and in turn to the nuclear structure of the atoms that make up nearly all visible matter in our universe.
Since the 1980s, scientists have known that quark and antiquark spins within a proton account for, at best, a quarter of the overall proton spin. RHIC, a Department of Energy Office of Science user facility for nuclear physics research at Brookhaven National Laboratory, was built in part so that scientists could measure the contributions of other components, including different "flavors" of antiquarks (up and down) and the gluons that "glue" the quarks together to form particles such as protons and neutrons.
The antiquarks are particularly elusive. They form when gluons split into a sea of quark-antiquark pairs, but for some still unknown reason, more down antiquarks are found in the proton than up antiquarks! RHIC physicists have now found an opposite effect for the proton's spin! They found that the spins of up antiquarks make a greater contribution to overall proton spin than the spins of down antiquarks. This exciting new result provides a substantially deeper understanding of the quantum fluctuations of quarks and gluons inside the proton and the complexity of the proton spin puzzle. It also lays the groundwork for new experiments to probe deeper into the internal structure and properties of nucleons (that is, protons and neutrons).
STAR Collaboration Co-spokesperson, Brookhaven National Laboratory
Research at the Relativistic Heavy Ion Collider, a scientific user facility, is funded primarily by the Department of Energy (DOE), Office of Science, Nuclear Physics. The STAR endcap calorimeter used for the forward and rearward measurements included in this analysis was funded in large part by the National Science Foundation. In addition, the team used the National Energy Research Scientific Computing Center, a scientific user facility at Lawrence Berkeley National Laboratory, funded by the DOE Office of Science, Advanced Scientific Computing Research. This work was supported in part by the Office of Nuclear Physics within the DOE Office of Science, the U.S. National Science Foundation, the Ministry of High Education and Science of the Russian Federation, National Natural Science Foundation of China, Chinese Academy of Science, the Ministry of Science and Technology of China and the Chinese Ministry of Education, the National Research Foundation of Korea, Czech Science Foundation and Ministry of Education, Youth and Sports of the Czech Republic, Department of Atomic Energy and Department of Science and Technology of the Government of India, the National Science Centre of Poland, the Ministry of Science, Education and Sports of the Republic of Croatia, RosAtom of Russia and German Bundesministerium für Bildung, Wissenschaft, Forschung and Technologie and the Helmholtz Association.
J. Adam, et al. (STAR Collaboration), "Measurement of the longitudinal spin asymmetries for weak boson production in proton-proton collisions at √s=510 GeV." Physical Review D 99, 051102(R) (2019). [DOI: 10.1103/PhysRevD.99.051102]
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