Recent discoveries by Belle and BESIII of charged exotic quarkonium-like resonances provide fresh impetus for study of heavy exotic hadrons. In the limit Nc → ∞, MQ → ∞ the \documentclass[12pt]{minimal}
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\begin{document}$ \overline{Q}Q\overline{q}{q^{\prime }} $\end{document} tetraquarks (TQ-s) are expected to be narrow and slightly below or above the \documentclass[12pt]{minimal}
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\begin{document}$ \overline{Q}{q^{\prime }} $\end{document} and \documentclass[12pt]{minimal}
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\begin{document}$ Q\overline{q} $\end{document} two-meson threshold. The isoscalar TQ-s manifest themselves by decay to \documentclass[12pt]{minimal}
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\begin{document}$ \left( {\overline{Q}Q} \right) $\end{document}ππ, and the ~30 MeV heavier charged isotriplet TQ-s by decays into \documentclass[12pt]{minimal}
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\begin{document}$ \left( {\overline{Q}Q} \right) $\end{document}π . The new data strongly suggest that the real world with Nc = 3, Q = c, b and q,q′ = u,d is qualitatively described by the above limit. We discuss the relevant theoretical estimates and suggest new signatures for TQ-s in light of the recent discoveries. We also consider “baryon-like” states \documentclass[12pt]{minimal}
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\begin{document}$ Q{Q^{\prime }}\overline{q}{{\overline{q}}^{\prime }} $\end{document}, which if found will be direct evidence not just for near-threshold binding of two heavy mesons, but for genuine tetraquarks with novel color networks. We stress the importance of experimental search for doubly-heavy baryons in this context.
