A precision measurement of the W boson decay branching fractions in pp collisions at $\sqrt{s} = 13~\mathrm{TeV}$

The leptonic and inclusive hadronic decay branching fractions of the W boson are studied using $35.9~\mathrm{fb}^{-1}$ of proton-proton collision data collected at $\sqrt{s}=13~\mathrm{TeV}$ during the 2016 run of the CMS experiment. Events characterized by the production of two W bosons, or of a W boson accompanied by jets, are selected. Multiple event categories sensitive to the signal processes are defined based on the presence of energetic isolated charged leptons, the number of hadronic jets, and the number of b-tagged jets. A maximum likelihood estimate of the W branching fractions is carried out by fitting to the data in each event category simultaneously. The branching fractions of the W boson decaying into electron, muon, and tau lepton final states amount to $(10.83 \pm 0.1)\%$, $(10.94 \pm 0.08)\%$, and $(10.77 \pm 0.21)\%$, respectively, supporting the hypothesis of lepton universality for the weak interaction. Under the assumption of lepton universality, the inclusive leptonic and hadronic decay branching fractions are found to be $(10.89 \pm 0.08)\%$ and $(67.32 \pm 0.23)\%$, respectively. From these results, three standard model quantities are subsequently derived: the sum square of elements in the first two rows of the Cabibbo-Kobayashi-Maskawa (CKM) matrix $\sum{|\mathrm{V_{ij}}|^{2}} = 1.989 \pm 0.021$, the CKM element $|\mathrm{V_{cs}}| = 0.969 \pm 0.011$, and the strong coupling constant at the W mass scale, $\alpha_\mathrm{S}(m_\mathrm{W}) = 0.094 \pm 0.033$.