Hidden-Beauty Charged Tetraquarks and Heavy Quark Spin Conservation

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 non-conservaton in Zb,Zb′ decays into BB∗ and B∗B∗. Indications of possible signatures of the still missing Xb resonance are proposed.