An Improved determination of the width of the top quark

We present an improved determination of the total width of the top quark, Λt, using 5.4 fb-1 of integrated luminosity collected by the D0 Collaboration at the Tevatron p(bar p) Collider. The total width Λt is extracted from the partial decay width Λ(t → Wb) and the branching fraction Β(t → Wb). Λ(t → Wb) is obtained from the t-channel single top quark production cross section and Β(t → Wb) is measured in t(bar t) events. For a top mass of 172.5 GeV, the resulting width is Λt = 2.00-0.43+0.47 GeV. This translates to a top-quark lifetime of τt = (3.29-0.63+0.90) x 10-25 s. We also extract an improved direct limit on the CKM matrix element 0.81 < |Vtb| (le) 1 at 95% C.L. and a limit of |Vtb'| < 0.59 for a high mass fourth generation bottom quark assuming unitarity of the fourth generation quark mixing matrix.