2026/04/30 by Guang-Yu Wang, Xu-Chang Zheng, Guang-Zhi Xu
#hep-ph
In this paper, we calculate the decay widths and branching fractions for the decays Z → J/ψ+Υ(nS) (n=1,2,3) at future super Z factory and at the CEPC/FCC-ee, including both the relativistic and QCD corrections within the framework of nonrelativistic QCD. Both the relativistic and QCD corrections are found to be large and negative. Compared to the leading-order results, the decay widths are significantly reduced by the higher-order corrections due to significant numerical cancellations. Despite the resulting large theoretical uncertainties, sizable event rates from these rare decay channels could still be anticipated at future high-luminosity electron-positron colliders running around the Z-pole. Ultimately, our results provide an essential theoretical baseline, highlighting the necessity of incorporating even higher-order corrections and resummation techniques for future precision phenomenological studies.