Studies of $Z$ boson decay into double $\mathrm{{\rm Y}}$ mesons at the NLO QCD accuracy

In this paper, we employ the nonrelativistic QCD factorization to conduct a comprehensive examination of the $Z$ boson decay into a pair of $\mathrm{{\rm Y}}$ mesons, achieving accuracy at the next-to-leading-order (NLO) in ${\alpha }_s$. Our calculations demonstrate that the QED diagrams are indispensable in comparison to the pure QCD diagrams, and the implementation of QCD corrections markedly enhance the QCD results, whereas it substantially diminish the QED results. To ensure consistency with the experimental methodology, we have taken into account the feed-down transitions originating from higher excited states, which exhibit significant relevance. Combining all the contributions, we arrive at the NLO prediction of ${\mathcal{B}}_{Z\to \mathrm{{\rm Y}}(nS)+\mathrm{{\rm Y}}(mS)}\sim {10}^{-11}$, which is notably lower than the upper limits set by CMS.