Internal structure of newly discovered exotic heavy-quark hadrons

Determine whether the recently observed exotic hadrons containing charm quarks are predominantly compact multi-quark configurations (tetraquarks or pentaquarks), hadronic molecular bound states of conventional hadrons, or superpositions of both. This requires comprehensive amplitude analyses and complementary measurements capable of establishing their internal dynamics and isospin partners with high precision.

Background

LHCb, Belle II, and BESIII have reported a surge of exotic hadrons—states not fitting simple qq̄ or qqq classifications—particularly in amplitude analyses of B-hadron decays.

Their nature remains unsettled: competing interpretations include compact multiquark states and loosely bound hadronic molecules, with the possibility that different states or even a single state may exhibit mixed characteristics.

A high-luminosity Z-pole run would enable precision amplitude analyses (including modes with neutral final states) to search for isospin partners and refine the spectroscopy required to resolve these scenarios.

References

It is not clear whether they are compact objects of four or five quarks, or rather molecule-like bounds states of conventional hadrons, or even both.

— ECFA Higgs, electroweak, and top Factory Study  (2506.15390 - Abidi et al., 18 Jun 2025) in Section "Outlook on other avenues" (Flavour Physics)

The invariant mass fits in Sec.~\ref{sec:invariant-mass} favor the inclusion of the $\chi_{c1}\pi$--$D\ast\bar D\ast$ coupled-channel rescattering contribution. Nevertheless, the data precision of the present $m(\chi_{c1}\pi)$ spectra is insufficient to claim the establishment of either a $$ or a $$. Both spin hypotheses produce a narrow cusp-like enhancement at the $D\ast\bar D\ast$ threshold, whose direct measurement in the invariant-mass spectrum generally requires sufficiently fine mass bins.

— Implications of $B\to χ_{c1}πK$ data for an isovector $G$-odd $D^\ast\bar D^\ast$ molecular virtual state  (2608.18705 - Wang, 19 Aug 2026) in Section 4.2, subsection “cos theta_{K pi} distributions”; see also Section 3.2, subsection “Belle mass spectra”

However, the precise nature of the $T_{cc}(3875)+$, a molecule, a compact tetraquark, or an admixture thereof, still remains an open question.

— Magnetic Moments and Radiative Transitions of the $T_{cc}(3875)^+$ and its partner states  (2608.27961 - Wu et al., 28 Aug 2026) in Section 1, Introduction