Just as quantum electrodynamics describes how electrons are bound in atoms by the electromagnetic force, mediated by the exchange of photons, quantum chromodynamics (QCD) describes how quarks are bound inside hadrons by the strong force, mediated by the exchange of gluons. QCD seems to allow hadrons constructed from increasingly many quarks to exist, just as atoms with increasing numbers of electrons exist, yet such complex constructions seemed, until recently, not to be present in nature. Here we describe advances in the spectroscopy of mesons that are refining our understanding of the rules for predicting hadron structure from QCD.
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J.J.D. acknowledges support provided by US Department of Energy contract DE-AC05-06OR23177, under which Jefferson Science Associates manages Jefferson Laboratory and the Early Career Award contract DE-SC0006765. M.R.S. and R.E.M. are supported by US Department of Energy contract DE-FG02-05ER41374. M.R.S. acknowledges the Jefferson Science Associates Sabbatical Leave Support Program. We thank L. Weinstein for comments on the initial draft of this manuscript.
The authors declare no competing financial interests.
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Shepherd, M., Dudek, J. & Mitchell, R. Searching for the rules that govern hadron construction. Nature 534, 487–493 (2016). https://doi.org/10.1038/nature18011