Z_b(10610) and Z_b(10650) decays to bottomonium plus pion

We consider the transitions from the $Z_b(10610)$ and $Z_b(10650)$ resonances to the states of bottomonium with emission of a pion. The $Z_b$ resonances are viewed as `molecular' objects of a large spatial size made of heavy $B^{(*)}$ mesons, while the states of bottomonium are considered to be compact, so that an application of the leading (dipole) term in the QCD multipole expansion is assumed to be justified. In this way we calculate the ratios of the decay rates to the final states $Υ(nS)\, π$ with $n=1,2,3$, and the ratio of the decay rates to $h_b(kP)\, π$ with $k=1$ and 2. We find our estimates in a reasonable agreement with recent experimental data.

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