Insights into $Z_b(10610)$ and $Z_b(10650)$ from dipion transitions from $Upsilon(10860)$

The dipion transitions $Upsilon(10860)topi^+pi^-Upsilon(nS)$ ($n=1,2,3$) are studied in the framework of a unitary and analytic coupled-channel formalism previously developed for analysing experimental data on the bottomoniumlike states $Z_b(10610)$ and $Z_b(10650)$ [Phys. Rev. D 98, 074023 (2018)] and predicting the properties of their spin partners [Phys. Rev. D 99, 094013 (2019)]. In this work we use a relatively simple but realistic version of this approach, where the scattering and production amplitudes are constructed employing only short-ranged interactions between the open- and hidden-flavour channels consistent with the constraints from heavy quark spin symmetry, for an extended analysis of the experimental line shapes. In particular, the transitions from the $Upsilon(10860)$ to the final states $pi pi h_b(mP)$ ($m=1,2$) and $pi B^{(*)}bar B^* $ already studied before, are now augmented by the $Upsilon(10860)topi^+pi^-Upsilon(nS)$ final states ($n=1,2,3$). This is achieved by employing dispersion theory to account for the final state interaction of the $pipi$ subsystem including its coupling to the $Kbar K$ channel. Fits to the two-dimensional Dalitz plots for the $pi^+pi^-Upsilon$ final states were performed. Two real subtraction constants are adjusted to achieve the best description of the Dalitz plot for each $Upsilon(nS)$ $(n=1,2,3)$ while all the parameters related to the properties of the $Z_b$s are kept fixed from the previous study. A good overall description of the data for all $Upsilon(10860)topi^+pi^-Upsilon(nS)$ channels achieved in this work provides additional strong support for the molecular interpretation of the $Z_b$ states.