Searching for the rules that govern hadron construction

Quantum chromodynamics (QCD) describes how quarks get together to form hadrons. The most common hadrons are baryons, such as protons and neutrons, which consist of three quarks, and mesons, which consist of a quarkantiquark pair. But, in principle, QCD allows much more exotic constructions: particles consisting of many more quarks, or particles consisting of gluons, the carriers of the strong force, as well as quarks. Although examples of such exotic states have been found recently, experimental results and high-accuracy theoretical predictions from lattice QCD do not yield a clear pattern that would allow for deriving a rulebook for hadron construction from QCD. This Review discusses recent experimental and theoretical results obtained in the search for the rules that govern hadron construction, and the authors give their perspective on current and future research efforts in this area.