Measurement of B ( t → W b ) / B ( t → W q ) at s = 1.96 TeV

We present the measurement of R = B(t→W b )/B(t →W q ) in pp¯ collisions at √s = 1.96 TeV, using 230 pb .1 of data collected by the D∅ experiment at the Fermilab TevatronCollider. We fit simultaneously R and the number (Ntt¯ ) of selected top quarkpairs (tt¯), to the number of identified b-quark jets in events with one electron or one muon, three or more jets, and high transverse energy imbalance. To improve sensitivity, kinematical properties of events with no identified b-quark jets are included in the fit.We measure R = 1.03 +0.19 .0.17 (stat+syst), in good agreement with the standard model. We set lower limits of R >0.61 and |V tb | > 0.78 at 95% confidence level.