Measurement of B (t→Wb) / B (t→Wq) in Top-Quark-Pair Decays Using Dilepton Events and the Full CDF Run II Data Set

We present a measurement of the ratio of the top-quark branching fractions R=B(t?Wb)/B(t?Wq), where q represents any quark flavor, in events with two charged leptons, imbalance in total transverse energy, and at least two jets. The measurement uses proton-antiproton collision data at center-of-mass energy 1.96 TeV, corresponding to an integrated luminosity of 8.7??fb-1 collected with the Collider Detector at Fermilab during Run II of the Tevatron. We measure R to be 0.87±0.07, and extract the magnitude of the top-bottom quark coupling to be |Vtb|=0.93±0.04, assuming three generations of quarks. Under these assumptions, a lower limit of |Vtb|>0.85(0.87) at 95% (90%) credibility level is set.We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.This work was supported by the U.S. Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology ofJapan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A. P. Sloan Foundation; the Bundesministerium fürBildung und Forschung, Germany; the Korean World Class University Program, the National Research Foundation ofKorea; the Science and Technology Facilities Council and the Royal Society, United Kingdom; the Russian Foundation for Basic Research; the Ministerio de Ciencia e Innovación, and Programa Consolider-Ingenio2010, Spain; the Slovak R&D Agency; the Academy of Finland; the Australian Research Council (ARC); and theE U community Marie Curie Fellowship Contract No. 302103