An explanation of Belle states Z_b(10610) and Z_b(10650)

Understand

Belle report data on Upsilon (5S) -> Upsilon(1S,2S,3S)-piplus-piminus and chi_b(1P,2P)-piplus-piminus; they observe peaks in Upsilon-pi and chi_b-pi consistent with J^P=1^+.

• They interpret the peaks as molecular states Z_b(10608) and Z_b(10653).
• Their masses are just above Bbar-B^* and B^*bar-B^* thresholds at 10604.6 and 10650.2 MeV.
• An explanation in terms of cusps at these thresholds is presented here.