In the 1980s experiments performed by the European Muon Collaboration at CERN showed that the spin of the proton - that is its intrinsic angular momentum - could not be explained by simply adding together the spins of its constituent quarks (which have a magnitude of 1/2 in units of Planck's constant). Instead, researchers found that "up" and "down" quarks contributed less than 25% to the spin of the proton (which also has a magnitude of 1/2).
It has long been thought that the presence of strange quarks inside the proton might partly explain this "spin crisis". These quarks, which are heavier than up and down quarks, are typically observed only in high-energy cosmic rays or particle accelerators, not in the everyday nuclei that make up the world around us. However, the force that holds the quarks together inside protons and other particles is so strong that the uncertainty principle allows quark–antiquark pairs to spontaneously appear from the vacuum and then disappear a short time later.
The question is, do these ephemeral "sea quarks" contribute to the observed properties of the proton, such as its mass, charge, spin and magnetic moment? A series of experimental results, most recently from the Jefferson Laboratory in the US, now seems poised to provide a definitive answer to this question.
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