Multiquark States

Abstract

Why do we see certain types of strongly interacting elementary particles and not others? This question was posed over 50 years ago in the context of the quark model. M. Gell-Mann and G. Zweig proposed that the known mesons were $q\bar{q}$ and baryons $qqq$, with quarks known at the time $u$ ("up"), $d$ ("down"), and $s$ ("strange") having charges (2/3,-1/3,-1/3). Mesons and baryons would then have integral charges. Mesons such as $qq\bar{q}\bar{q}$ and baryons such as $qqqq\bar{q}$ would also have integral charges. Why weren't they seen? They have now been seen, but only with additional heavy quarks and under conditions which tell us a lot about the strong interactions and how they manifest themselves. The present article describes recent progress in our understanding of such "exotic" mesons and baryons.

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• Observation of the doubly charmed baryon ${\Xi }_{cc}^{++}$
• Prediction of narrow $N^{\ast }$ and ${\Lambda }^{\ast }$ resonances with hidden charm above 4 GeV
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• Heavy-quark symmetry implies stable heavy tetraquark mesons $Q_i{Q}_j{\bar{q}}_k{\bar{q}}_l$
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• Observation of a near-threshold enhancement in the e+e- -->Lambda_c+Lambda_c- cross section using initial-state radiation
• Baryons with two heavy quarks: Masses, production, decays, and detection
• Precise measurement of the $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}J/\psi$ cross section at center-of-mass energies from 3.77 to 4.60 GeV
• Are Y(4260) and {\rm Z${}_2^{+}$(4250)} ${\mathrm{D}}_1\mathrm{D}$ or ${\mathrm{D}}_0{\mathrm{D}}^{\ast }$ Hadronic Molecules?
• Observation of a new charged charmoniumlike state in B -> J/psi K pi decays
• Evidence of Two Resonant Structures in $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}{h}_c$
• Observation of anomalous Upsilon(1S)pi+pi- and Upsilon(2S)pi+pi- production near the Upsilon(5S) resonance
• $QQ\bar{Q}\bar{Q}$ states: masses, production, and decays
• Could the near-threshold $XYZ$ states be simply kinematic effects?
• A Theory of X and Z Multiquark Resonances
• What can radiative decays of the X(3872) teach us about its nature?
• Effects of S-wave thresholds
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• Charmonium resonances in the 3.9 GeV/$c^2$ energy region and the $X(3915)/X(3930)$ puzzle
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• Canonical description of the new LHCb resonances
• The New Sigma_b multiplet, heavy baryon mass predictions, meson-baryon universality and magnetic moments of Lambda_c and Lambda_b
• Revising the $f_1(1420)$ resonance
• Experimental Review on Pentaquarks
• $X(3872)$, $X_b$, and the ${\chi }_{b1}(3P)$ state
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