Current methods to extract the quark-mixing matrix element $|V_{ tb}|$ from single-top production measurements assume that $|V_{ tb}|\gg |V_{ td}|, |V_{ ts}|$: top quarks decay into $b$ quarks with 100% branching fraction, s-channel single-top production is always accompanied by a $b$ quark and initial-state contributions from $d$ and $s$ quarks in the $t$-channel production of single top quarks are neglected. Triggered by a recent measurement of the ratio $R=\frac{ |V_{ tb}|^{ 2}}{ |V_{ td}|^{ 2}+|V_{ ts}|^{ 2}+|V_{ tb}|^{ 2}}=0.90 \pm 0.04$ performed by the D0 collaboration, we consider a $|V_{ tb}|$ extraction method that takes into account non zero d- and s-quark contributions both in production and decay. We propose a strategy that allows to extract consistently and in a model-independent way the quark mixing matrix elements $|V_{ td}|$, $|V_{ ts}|$, and $|V_{ tb}|$ from the measurement of $R$ and from single-top measured event yields. As an illustration, we apply our method to the Tevatron data using a CDF analysis of the measured single-top event yield with two jets in the final state one of which is identified as a $b$-quark jet. We constrain the $|V_{ tq}|$ matrix elements within a four-generation scenario by combining the results with those obtained from direct measurements in flavor physics and determine the preferred range for the top-quark decay width within different scenarios.
Categories: hep-ph