Search for anomalous Wtb couplings in single top quark production

In 0.9 fb(-1) of pp collisions, the D0 Collaboration presented evidence for single top quarkproduction in events with an isolated lepton, missing transverse momentum, and two to four jets. We examine these data to study the Lorentz structure of the Wtb coupling. The standard model predicts a left-handed vector couplingat the Wtb vertex. The most general lowest dimension, CP-conserving Lagrangian admits right-handed vector and left- or right-handed tensor couplingsas well. We find that the data prefer the left-handed vector couplingand set upper limits on the anomalous couplings. These are the first direct constraints on a general Wtb interaction and the first direct limitson left- and right-handed tensor couplings.