Improved determination of the width of the top quark

Abstract

We present an improved determination of the total width of the top quark, $\Gamma_t$, using $5.4 \text{fb}^{−1}$ of integrated luminosity collected by the D0 Collaboration at the Tevatron $p\bar p$ Collider. The total width $\Gamma_t$ is extracted from the partial decay width $\Gamma(t\rightarrow Wb)$ and the branching fraction $\mathscr{B}(t→Wb). \Gamma(t→Wb)$ is obtained from the t-channel single top-quark production cross section and $\mathscr{B}(t→Wb)$ is measured in $t\bar t$ events. For a top mass of 172.5 GeV, the resulting width is $\Gamma_t=2.00^{+0.47}_{−0.43} \text{GeV}$. This translates to a top-quark lifetime of $\tau_t=(3.29^{+0.90}_{−0.63})×10^{−25} s$. We also extract an improved direct limit on the Cabibbo-Kobayashi-Maskawa quark-mixing matrix element $0.81<|V_{tb}|≤1$ at 95% C.L. and a limit of $|V_{tb^′}|<0.59$ for a high-mass fourth-generation bottom quark assuming unitarity of the fourth-generation quark-mixing matrix.