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In recent years there has been a rapidly growing body of experimentalevidence for existence of exotic, multiquark hadrons, i.e. mesons which containadditional quarks, beyond the usual quark-antiquark pair and baryons whichconsist of more than three quarks. In all cases with robust evidence theycontain at least one heavy quark Q=c or b, the majority including two heavyquarks. Two key theoretical questions have been triggered by these discoveries:(a) how are quarks organized inside these multiquark states -- as compactobjects with all quarks within one confinement volume, interacting via colorforces, perhaps with an important role played by diquarks, or as deuteron-likehadronic molecules, bound by light-meson exchange? (b) what other multiquarkstates should we expect? The two questions are tightly intertwined. Each of theinterpretations provides a natural explanation of parts of the data, butneither explains all of the data. It is quite possible that both kinds ofstructures appear in Nature. It may also be the case that certain states aresuperpositions of the compact and molecular configurations. This Whitepaperbrings together contributions from many leading practitioners in the field,representing a wide spectrum of theoretical interpretations. We discuss theimportance of future experimental and phenomenological work, which will lead tobetter understandingof multiquark phenomena in QCD.

Substructure of Multiquark Hadrons (Snowmass 2021 White Paper)