Direct CKM determination from W decays at future lepton colliders

Abstract We project the reach of future lepton colliders for measuring CKM elements from direct observations of W decays. We focus our attention to |V cs | and |V cb | determinations, using FCC-ee as case study. We employ state-of-the-art jet flavor taggers to obtain the projected sensitivity, and scan over tagger performances to show their effect. We conclude that future lepton collider can sizeably improve the sensitivity on |V cs | and |V cb |, albeit the achievable reach will strongly depend on the level of systematic uncertainties on tagger parameters.