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At first glance, the pion seems simple. It’s the lightest meson, made up of an up quark and a down quark. But like everything in quantum chromodynamics (QCD)—the theory of the strong force—things get more complicated the closer you look. The pion, as it turns out, contains not just two “valence” quarks but also a “sea” of virtual quarks that pop in and out of existence. In addition, the pion hosts gluons, which are the carriers of the strong force that binds quarks together (see 6 March 2017 Viewpoint). Previous work suggested that gluons contribute only 10% of the pion’s momentum at typical experimental energies of a few GeV, but a new study incorporating old and new data has found that the gluon fraction is 30%.

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