Measurement of the ratio B(t to Wb)/B(t to Wq) in pp collisions at sqrt(s) = 8 TeV

Understand

The ratio of the top-quark branching fractions R = B(t to Wb)/B(t to Wq), where the denominator includes the sum over all down-type quarks (q = b, s, d), is measured in the t t-bar dilepton final state with proton-proton collision data at sqrt(s) = 8 TeV from an integrated luminosity of 19.7 inverse femtobarns, collected with the CMS detector.

• In order to quantify the purity of the signal sample, the cross section is measured by fitting the observed jet multiplicity, thereby constraining the signal and background contributions.
• By counting the number of b jets per event, an unconstrained value of R = 1.014 +/- 0.003 (stat.) +/- 0.032 (syst.) is measured, in good agreement with current precision measurements in electroweak and flavour sectors.
• A lower limit R > 0.955 at the 95% confidence level is obtained after requiring R <= 1, and a lower limit on the Cabibbo-Kobayashi-Maskawa matrix element abs(V[tb]) > 0.975 is set at 95% confidence level.