Just as Quantum Electrodynamics describes how electrons are bound in atoms by the electromagnetic force, mediated by exchange of photons, Quantum Chromodynamics (QCD) describes how quarks are bound inside hadrons by the strong force, mediated by exchange of gluons. At face value, QCD allows hadrons constructed from increasingly many quarks to exist, just as atoms with increasing numbers of electrons exist, yet such complex constructions seemed, until recently, to not be present in nature. In what follows we describe advances in the spectroscopy of mesons that are refining our understanding of the rules for building hadrons from QCD.
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