Assuming the dominance of the spin-spin interaction in a diquark, we point out that the mass difference in the beauty sector M(Zb^')^±-M(Zb)^± scales with quark masses as expected in QCD, with respect to the corresponding mass difference M(Zc^')^±-M(Zc)^±. Notably, we show that the decays Υ(10890)→Υ(nS)π⁺π^- and Υ(10890)→(hb(1P),hb(2P))π⁺π^- are compatible with heavy quark spin conservation if the contributions of Zb,Zb^' intermediate states are taken into account, Υ(10890) being either a Υ(5S) or the beauty analog of Yc(4260). Belle results on these decays support the quark spin wave function of the Z states as tetraquarks. We also consider the role of light quark spin nonconservaton in Zb,Zb^' decays into BB* and B*B*. Indications of possible signatures of the still missing Xb resonance are proposed.
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Ali et al. (2015) studied this question.