αs and |Vcs| determination, and CKM unitarity test, from W decays at NNLO

The hadronic (ΓhadW) and total (ΓtotW) widths of the W boson, computed at least at next-to-next-to-leading-order (NNLO) accuracy, are combined to derive a new precise prediction for the hadronic W branching ratio BhadW≡ΓhadW/ΓtotW=0.682±0.011par, using the experimental Cabibbo-Kobayashi-Maskawa (CKM) matrix elements, with uncertainties dominated by the input parameters of the calculations, or BhadW=0.6742±0.0002th±0.0001par assuming CKM unitarity. Comparing the theoretical predictions and experimental measurements for various W decay observables, the NNLO strong coupling constant at the Z pole, αs(mZ2)=0.117±0.042exp±0.004th±0.001par, as well as the charm-strange CKM element, |Vcs|=0.973±0.004exp±0.002par, can be extracted under different assumptions. We also show that W decays provide today the most precise test of CKM unitarity for the 5 quarks lighter than mW, ∑u,c,d,s,b|Vij|2=1.999±0.008exp±0.001th. Perspectives for αs and |Vcs| extractions from W decays measurements at the LHC and future e+e− colliders are presented.