| Abstract: | Recent experimental data on the \({\Upsilon(4S)\to\Upsilon(1S)\eta}\) and \({\Upsilon(4S)\to h_{b}(1P)\eta}\) processes seem to contradict the naive expectation that hadronic transitions with spin-flipping terms should be suppressed with respect those without spin-flip. We analyze these transitions using the QCD multipole expansion (QCDME) approach and within a constituent quark model framework that has been applied successfully to the heavy-quark sectors during the last years. The QCDME formalism requires the computation of hybrid intermediate states which has been performed in a natural, parameter-free extension of our constituent quark model based on the quark confining string (QCS) scheme. We show that (i) the M1–M1 contribution in the decay rate of the \({\Upsilon(4S)\to\Upsilon(1S)\eta}\) is important and its suppression until now is not justified; (ii) the role played by the \({L=0}\) hybrid states, which enter in the calculation of the M1–M1 contribution, explains the observed enhancement in the \({\Upsilon(4S)\to\Upsilon(1S)\eta}\) decay width; and (iii) the anomalously large decay rate of the \({\Upsilon(4S)\to h_{b}(1P)\eta}\) transition has the same physical origin. |
