Abstract
The ratio of the top-quark branching fractions R=B(t→Wb)/B(t→Wq), where the denominator includes the sum over all down-type quarks (q=b,s,d), is measured in the tt¯ dilepton final state with proton–proton collision data at s=8 TeV from an integrated luminosity of 19.7 fb−1, 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 a 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 |Vtb|>0.975 is set at 95% confidence level. The result is combined with a previous CMS measurement of the t-channel single-top-quark cross section to determine the top-quark total decay width, Γt=1.36±0.02(stat.)−0.11+0.14(syst.) GeV.
Highlights
Because of its large mass [1], the top quark decays before fragmenting or forming a hadronic bound state [2]
In this Letter, we present a measurement of R = B(t → Wb)/B(t → Wq), where the denominator includes the sum over the branching fractions of the top quark to a W boson and a down-type quark (q = b, s, d)
Since the estimate is obtained for a specific scenario in which R = 1, an extra linear dependence of kst on R is introduced in order to account for the increase in the tW cross section as |V tb| becomes smaller while |V td| and |V ts| become larger [4]
Summary
Because of its large mass [1], the top quark decays before fragmenting or forming a hadronic bound state [2]. The indirect measurement of |V tb|, from the unitarity constraint of the CKM matrix, is |V tb| = 0.999146+−00..000000002416 [3] Any deviation from this value or in the partial decay width of the top quark to b quarks, would indicate new physics contributions such as those from new heavy up- and/or down-type quarks or a charged Higgs boson, amongst others [4]. Direct searches at the Large Hadron Collider (LHC) have set lower limits on the mass of these hypothetical new particles [5,6,7,8,9,10,11,12,13,14,15], and the observation of a SM Higgs boson candidate [16,17,18] places stringent constraints on the existence of a fourth sequential generation of quarks These results support the validity of both the. The measurement of R is combined with a previously published CMS result of the t-channel production cross section of single top quarks in pp collisions [25] to yield an indirect determination of the top-quark total decay width
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