Measurement of CKM element $|V_{cb}|$ from $W$ boson decays at the future Higgs factories

TL;DR

This paper evaluates the potential for future Higgs factories to precisely measure the CKM matrix element |V_{cb}| using semileptonic WW decays, projecting uncertainties below 1%.

Contribution

It provides detailed simulation-based projections of |V_{cb}| measurement precision at future Higgs factories, including systematic uncertainty considerations.

Findings

Projected statistical uncertainties below 1% for |V_{cb}| measurement.

Systematic uncertainties, especially flavor-tagging, are significant but can be reduced.

Future colliders can achieve high-precision CKM element measurements.

Abstract

This study investigates the precision measurement of the CKM matrix element $|V_{cb}|$ through semileptonic $WW$ events at future Higgs factories, $i.e.$, FCC-ee, ILC, C$^3$, and CEPC. We use full detector simulation to generate the $WW \to \ell \nu cb$ signal events and various backgrounds at $\sqrt{s} = 240$ 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.