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Accepted · first revision · record

Title: Are near-threshold exotic hadrons compact multiquarks or hadronic molecules?
Plain: Many exotic hadrons have masses very close to the combined mass of two ordinary hadrons, which suggests a loosely bound molecule of two mesons or of a meson and a baryon. Others argue they are compact objects in which four or five quarks are tightly bound together.
Precise: For chi_c1(3872), $Z_c(3900)$, $T_{cc}+(3875)$, $P_c(4312)$, $P_c(4440)$ and $P_c(4457)$, determine the structure: the weight of the two-hadron component (compositeness, between 0 and 1) versus compact diquark-antidiquark or pentaquark components. Answer: a classification supported by pole positions, line shapes, production rates in high-multiplicity pp collisions, and lattice QCD finite-volume spectra.
Settled by: Lattice QCD coupled-channel scattering at physical quark masses giving pole positions and residues, matched to high-statistics experimental line shapes.
Kind: phenomenon
Answer type: classification
References: (empty)
Literature status: contested
Status note: The LHCb pentaquarks (2019) and $T_{cc}+$ (2021) are commonly described as molecules, but compact interpretations persist and no single observable discriminated between them as of 2026.
Posed since: 2003