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

The mixing of S-D partial waves for the heavy meson pairs in the decays Υ(5S)→[B*B̄+c.c.]π and Υ(5S)→B*B ^̄*π 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∼15 MeV. It is also argued that the mixing is likely to reach maximum at an energy approximately equal to the mass gap between the thresholds.