Enhanced mixing of partial waves near threshold for heavy meson pairs and properties of $Z_b(10610)$ and $Z_b(10650)$ resonances

Abstract

The mixing of $S-D$ partial waves for the heavy meson pairs in the decays ${\rm Y}(5S)\to [B^{\ast }\bar{B}+c.c.]\pi$ and ${\rm Y}(5S)\to B^{\ast }{\bar{B}}^{\ast }\pi$ is considered. It is argued that this mixing, enhanced by the heavy meson mass, is calculable as dominated by a rescattering through pion exchange if the production of the heavy mesons is dominated by $S$-wave `molecular' resonances $Z_b(10610)$ and $Z_b(10650)$. The effect of the mixing grows with the c.m. energy $E$ in each channel over the threshold, and may reach $10÷20$% in the rate at the upper end of applicability of the discussed approach, $E\sim 15$MeV. It is also argued that the mixing is likely to reach maximum at energy approximately equal to the mass gap between the thresholds.

Comments: 8 pages, 3 figures