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

Belle report data on Gamma(5,S) -> Gamma(1S, 2S, 3S)pi(+)pi(-) and chi(b)(1P, 2P)pi(+)pi(-); they observe peaks in Gamma 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 (B) over barB* and (B) over bar *B* thresholds at 10604.6 and 10650.2MeV. An explanation in terms of cusps at these thresholds is presented here. The product of the rising phase space for (B) over barB* and (B) over bar *B* with the cusps creates peaks a few MeV higher in (B) over barB* and (B) over bar *B*, and these peaks can de-excite to pi(+)pi(-)Gamma(1S, 2S, 3S) and pi(+)pi(-) chi(b)(1P, 2P). Copyright (C) EPLA, 2011