The role of spin-flipping terms in hadronic transitions of ${\rm Y}(4S)$

Abstract

Recent experimental data on the ${\rm Y}(4S)\to {\rm Y}(1S)\eta$ and ${\rm Y}(4S)\to h_b(1P)\eta$ 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 ${\rm Y}(4S)\to {\rm Y}(1S)\eta$ is important and its supression 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 enhancement in the ${\rm Y}(4S)\to {\rm Y}(1S)\eta$ decay rate; and iii) the anomalously large decay rate of the ${\rm Y}(4S)\to h_b(1P)\eta$ process has the same physical origin.

Comments: 9 pages, 1 figure, 4 tables