Abstract
Assuming the dominance of the spin-spin interaction in a diquark, we point out that the mass difference in the beauty sector M(Z_b^prime) - M(Z_b) scales with quark masses as expected in QCD, with respect to the corresponding mass difference M(Z_c^prime) - M(Z_c). Notably, we show that the decays Upsilon(10890) --> Upsilon(nS) pi pi and Upsilon(10890) --> (h_b(1P), h_b(2P)) pi pi are compatible with heavy-quark spin conservation if the contributions of Z_b,Z_b^prime intermediate states are taken into account, Upsilon(10890) being either a Upsilon(5S) or the beauty analog of Y_c(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 non-conservaton in Z_b,Z_b^prime decays into B B^* and B^* B^*. Indications of possible signatures of the still missing X_b resonance are proposed.
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