Dynamical Picture for the Formation and Decay of the Exotic $XYZ$ Mesons

We present a new dynamical picture that identifies the formation of the exotic $\overline{c}c$-containing states $XYZ$ with the confinement-induced hadronization of a rapidly separating pair of a compact diquark and antidiquark. This picture combines the advantages of diquark-based models, which can accommodate much of the known $XYZ$ spectrum, with the experimental fact that such states are both relatively narrow and are produced promptly. It also naturally explains the preference of some of the exotic states to decay to $\psi (2S)$, rather than $J/\psi$, in terms of a simple wave-function overlap effect.

Figure 1

Diquark-antidiquark production. The arrows indicate a color flux tube connecting the color-$\overline{\mathbf{3}}$ diquark $\delta$ and the color-$\mathbf{3}$ antidiquark $\overline{\delta }$.

Figure 2

Illustration of the production of a spatially extended diquark-antidiquark state $\delta \overline{\delta }$ attracted by long-range color forces (indicated by gluon lines). Here, the mechanism is illustrated for $B^0\to Z(4430{)}^{-}{K}^{+}$, where the filled square indicates the $\overline{b}$-quark weak decay.

Figure 3

The baryonium system: Fragmentation of the color flux tube connecting the diquark-antidiquark pair $\delta \overline{\delta }$ into the lowest-lying baryon-antibaryon state, ${\mathrm{\Lambda }}_c^{+}{\overline{\mathrm{\Lambda }}}_c^{-}$.

Figure 4

Illustration of the mechanism that suppresses the hadronization rate of diquark-antidiquark ($\delta \overline{\delta }$) extended states, due to the small wave-function overlap (indicated by dashed lines) at large spatial separation.