The $Z_b(10610)^\pm$ and $Z_b(10650)^\pm$ are two bottomonium-like structures discovered in the $\pi h_b(mP)$, $\pi \Upsilon(nS)$ and $B^\ast\bar B^{ (\ast)}+h.c.$ invariant mass spectra, where $m=\{ 1,2\}$ and $n=\{ 1,2,3\}$. Their nature is puzzling due to their charge, which forces its minimal quark content to be $b\bar b u\bar d$ ($b\bar b d\bar u$). Thus, it is necessary to explore four-quark systems in order to understand their inner structure. Additionally, their strong coupling to channels such as $\pi \Upsilon$ and the closeness of their mass to $B^\ast\bar B^{ (\ast)}$-thresholds stimulates a molecular interpretation. Within the framework of a constituent quark model which satisfactorily describes a wide range of properties of (non-)conventional hadrons containing heavy quarks, we perform a coupled-channels calculation of the $I^G(J^{ PC})=1^+(1^{ +-})$ hidden-bottom sector including $B^{ (\ast)}\bar B^{ \ast}+h.c.$, $\pi h_b$, $\pi \Upsilon$ and $\rho\eta_b$ channels. We analyze the line shapes in the different channels, describing the $\Upsilon(5S)\to \pi B^{ (*)}\bar B^{ (*)}$ by means of the $^3P_0$ model. Since our description of the line shapes promising, we perform the same coupled-channels calculation for the $Z_b$'s with $J^{ --}$, where $J=\{ 0,1,2\}$. This allows us to obtain a fair description of the corresponding line shapes. The study of the analytic structure of the $S$-matrix suggests that the experimental $Z_b$ structures arise as a combination of several poles with $J^{ PC}=0^{ --}$, $1^{ \pm-}$ and $2^{ --}$ quantum numbers nearby the $B\bar B^\ast$ and $B^\ast\bar B^\ast$ thresholds.
Categories: hep-ph