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This study investigates the precision measurement of the CKM matrix element |V₂₁| through semileptonic WW events at future Higgs factories, i. e. , FCC-ee, ILC, C³, and CEPC. We use full detector simulation to generate the WW cb signal events and various backgrounds at s = 240 GeV with unpolarized beams. The relative statistical uncertainties are projected to be 0. 91\% for the muon channel and 1. 2\% for the electron channel, assuming a baseline integrated luminosity of 5 ab^-1. The sensitivities at other Higgs factory scenarios are also projected. Possible contributors to systematic uncertainties are discussed, with the most prominent one being the systematics of flavor-tagging and mistagging rates. Combining with WW threshold runs, the relative systematic uncertainty can be further reduced.
Liang et al. (Mon,) studied this question.