Are near-threshold exotic hadrons compact multiquarks or hadronic molecules?
In plain words
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 statement
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.
What would settle it
Lattice QCD coupled-channel scattering at physical quark masses giving pole positions and residues, matched to high-statistics experimental line shapes.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
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.
Problems built on this one
Special cases
- Spin and parity of the Pc(4440) and Pc(4457) pentaquarks · open
- Pole of the doubly charmed $T_{cc}+$ from lattice QCD · open
- Compositeness of chi_c1(3872) from its line shape · open
Results
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