The Z_b(10610) and Z_b(10650) as axial-vector tetraquark states in the QCD sum rules

In this article, we study the axial-vector mesons $Z_b(10610)$ and $Z_b(10650)$ with the $C\gamma_\mu-C\gamma_5$ type and $C\gamma_\mu-C\gamma_\nu$ type interpolating currents respectively by carrying out the operator product expansion to the vacuum condensates up to dimension-10. In calculations, we explore the energy scale dependence of the QCD spectral densities of the hidden bottom tetraquark states in details for the first time, and suggest a formula $\mu=\sqrt{M^2_{X/Y/Z}-(2{\mathbb{M}}_b)^2}$ with the effective mass ${\mathbb{M}}_b=5.13\,\rm{GeV}$ to determine the energy scales. The numerical results favor assigning the $Z_b(10610)$ and $Z_b(10650)$ as the $C\gamma_\mu-C\gamma_5$ type and $C\gamma_\mu-C\gamma_\nu$ type hidden bottom tetraquark states, respectively. We obtain the mass of the $J^{PC}=1^{++}$ hidden bottom tetraquark state as a byproduct, which can be compared to the experimental data in the futures. Furthermore, we study the strong decays $Z_b^\pm(10610)\to\Upsilon\pi^{\pm}\, ,\,\eta_b\rho^{\pm}$ with the three-point QCD sum rules, the decay widths also support assigning the $Z_b(10610)$ as the $C\gamma_\mu-C\gamma_5$ type hidden bottom tetraquark state.