Evidence for Single Top Production at the D0 Experiment

Top quarks were first observed in ttbar pair production in 1995.  Ever since, the Tevatron experiments have been looking for the electroweak production of single top quarks, also produced by the standard model.  Single top quark events can be used to study the Wtb coupling, to measure the magnitude of the CKM matrix element Vtb without assuming only three quark generations, or as a source of polarized top quarks.

I will present the first evidence for the production of single top quarks with the D0 detector at the Fermilab Tevatron proton-antiproton collider.  Individual top quarks are expected to be produced in association with bottom quarks through the exchange of a W boson (tb channel), or via the W-gluon fusion process (tgb channel).  After applying selection criteria to the data, the signal-to-background ratio is improved with an algorithm to identify jets originating from a b quark.  On the remaining data, several multivariate techniques are used for both production channels.  I will describe the background model and analysis techniques.  The most sensitive analysis, using boosted decision trees, give a cross section sigma (ppbar --> tb+X, tqb+X) = 4.9 +/- 1.4 pb with a significance of 3.4 standard deviations.  The cross section measurement is used to directly determine for the first time the CKM matrix element Vtb without requiring 3 families of quarks or CKM matrix unitarity.