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

Understand

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 $\Upsilon(nS)\, \pi$ with $n=1,2,3$, and the ratio of the decay rates to $h_b(kP)\, \pi$ with $k=1$ and 2.
• We find our estimates in a reasonable agreement with recent experimental data.