Particle Physics: Quark Studies Put Theorists in a Spin

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Science  16 Feb 1996:
Vol. 271, Issue 5251, pp. 911
DOI: 10.1126/science.271.5251.911


Amsterdam, the Netherlands—Eight years ago, a group of European particle physicists studying the internal structure of protons and neutrons—the building blocks of atomic nuclei collectively known as nucleons—made a startling discovery: The three valence quarks within each nucleon, which define its physical properties, make only a small contribution to its spin. Theorists had long assumed that they defined the spin entirely. Other groups quickly came up with broadly similar conclusions, although their results were sufficiently divergent that many physicists believed there was something wrong with the experiments. Now there's no doubt: Three groups are all reporting that the valence quarks contribute only about 25% of the nucleons' spin. Theorists now consider gluons the prime candidates for the major spin-carriers.