Abstract
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.
Highlights
During almost two decades, tens of charmonium and bottomoniumlike states, the so-called XYZ mesons, have been observed at B-factories (BABAR, Belle, and CLEO), τ-charm facilities (CLEO-c and BESIII), and proton(anti)proton colliders (CDF, D0, LHCb, ATLAS, and CMS)
It is worth emphasizing that heavy flavor symmetry dictates the BðÃÞB à and DðÃÞD à interactions are practically identical. This happens because the BðÃÞ and DðÃÞ wave functions are similar and, regarding the meson-meson interaction, the heavy quark c, or b, acts as an spectator and the dynamics is governed by the light-quark pair
In order to clarify their inner structure, within the framework of a constituent quark model that satisfactorily describes a wide range of properties ofconventional hadrons containing heavy quarks, we performed in Ref. [8] a coupled-channels calculation of the IGðJPCÞ 1⁄4 1þð1þ−Þ ccud sector around the mass energies of the Zcð3900ÞÆ=Zcð3885ÞÆ and Zcð4020Þ signals
Summary
Tens of charmonium and bottomoniumlike states, the so-called XYZ mesons, have been observed at B-factories (BABAR, Belle, and CLEO), τ-charm facilities (CLEO-c and BESIII), and proton(anti)proton colliders (CDF, D0, LHCb, ATLAS, and CMS). The common characteristic of most of these states is that, their quantum numbers are compatible with naive QQstates (Q either c- or b-quark), their masses, static properties, and decay patterns point to more complex structures involving higher Fock-state components. It was not until 2011 when undeniable evidence of exotic mesons with forbidden quantum numbers for a quark-antiquark pair were observed.
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