We examine the four-quark structure of the recently discovered charged $Z_c(3900)$, $Z(4430)$, and $X_b(5568)$ states. We calculate the widths of the strong decays $Z_c^{+}\to J/\psi {\pi }^{+}$ (${\eta }_c{\rho }^{+}$, ${\overline{D}}^0{D}^{*+}$, ${\overline{D}}^{*0}{D}^{+}$), $Z(4430{)}^{+}\to J/\psi {\pi }^{+}$ ($\psi (2s){\pi }^{+}$), and $X_b^{+}\to B_s{\pi }^{+}$ within a covariant quark model previously developed by us. We find that the tetraquark-type current widely used in the literature for the $Z_c(3900)$ leads to a significant suppression of the $\overline{D}{D}^{*}$ and ${\overline{D}}^{*}D$ modes. Contrary to this a molecular-type current provides an enhancement by a factor of 6–7 for the $\overline{D}{D}^{*}$ modes compared with the $Z_c^{+}\to J/\psi {\pi }^{+}$, ${\eta }_c{\rho }^{+}$ modes in agreement with recent experimental data from the BESIII Collaboration. In the case of the $Z(4430)$ state we test a sensitivity of the ratio $R_Z$ of the $Z(4430{)}^{+}\to \psi (2s){\pi }^{+}$ and $Z(4430{)}^{+}\to J/\psi {\pi }^{+}$ decay rates to a choice of the size parameter ${\mathrm{\Lambda }}_{Z(4430)}$ of the $Z(4430)$. Using the upper constraint for the sum of these two modes deduced from the LHCb Collaboration data we find that $R_Z$ varies from 4.64 to 4.08 when ${\mathrm{\Lambda }}_{Z(4430)}$ changes from 2.2 to 3.2 GeV. Also we make the prediction for the $Z(4430{)}^{+}\to D^{*+}{\overline{D}}^{*0}$ decay rate.

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• Particle interactions