The hadronic ($\Gamma^{ \rm W}_{ \rm had}$) and total ($\Gamma^{ \rm W}_{ \rm tot}$) 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 ${ \cal B}^{ \rm W}_{ \rm had} \equiv \Gamma^{ \rm W}_{ \rm had}/\Gamma^{ \rm W}_{ \rm tot}$ = $0.682 \pm 0.011_{ \rm par}$, using the experimental Cabibbo-Kobayashi-Maskawa (CKM) matrix elements, or ${ \cal B}^{ \rm W}_{ \rm had} = 0.6742 \pm 0.0002_{ \rm th} \pm 0.0001_{ \rm par}$ assuming CKM unitarity, with uncertainties dominated by the input parameters of the calculations. Comparing the theoretical predictions and experimental measurements for various W decay observables, the NNLO strong coupling constant at the Z pole, $\alpha_s(m_Z) = 0.117 \pm 0.042_{ \rm exp} \pm 0.004_{ \rm th} \pm 0.001_{ \rm par}$, as well as the charm-strange CKM element, $\rm V_{ cs}$ = 0.973 $\pm$ 0.004$_{ \rm exp}$ $\pm$ 0.002$_{ \rm par}$, 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 $m_{ \rm W}$, $\sum_{ u,c,d,s,b} |V_{ ij}|^2 = 1.999 \pm 0.008_{ \rm exp} \pm 0.001_{ \rm th}$. Perspectives for $\alpha_s$ and $\rm V_{ cs}$ extractions from W decays measurements at the LHC and future $e^+e^-$ colliders are presented.
Categories: hep-ph hep-ex