Analysis of double-$J/\psi$ production in $Z$ decay at next-to-leading-order QCD accuracy

TL;DR

This paper provides a detailed NLO QCD analysis of double-$J/ar{J}$ production in Z decay, revealing that combined QCD and QED effects significantly enhance the predicted branching ratio, which remains below experimental upper limits.

Contribution

It offers the first comprehensive NLO QCD calculation including QED effects for double-$J/ar{J}$ production in Z decay, improving theoretical predictions.

Findings

QCD corrections increase the branching ratio

QED contributions reduce the QCD results

Predicted branching ratio is below experimental upper limits

Abstract

In this article, we study in detail the double-$J/\psi$ yield through $Z$ decay at the next-to-leading-order (NLO) QCD accuracy within the nonrelativistic QCD factorization. At the tree level, the pure QCD diagrams predict a branching ratio of $\mathcal{B}_{Z \to J/\psi+J/\psi} \sim 10^{-12}$; however, the inclusion of the QED diagrams would augment this prediction by approximately 2-3 orders of magnitude. After incorporating the QCD corrections, the QCD results exhibit a considerable increase, whereas the QED results undergo a substantial reduction. Combining the QCD and QED contributions at NLO in $\alpha_s$, it is observed that the prediction of $\mathcal{B}_{Z \to J/\psi+J/\psi}=(1.110^{+0.334+0.054}_{-0.241-0.001})\times 10^{-10}$, which displays a fairly steady dependence on the renormalization scale, is significantly lower than the upper limits released by CMS.