Search for Anomalous $Wtb$ Couplings in Single Top Quark Production

In $0.9{\mathrm{fb}}^{-1}$ of $p\overline{p}$ collisions, the D0 Collaboration presented evidence for single top quark production 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 coupling at the $Wtb$ vertex. The most general lowest dimension, $CP$-conserving Lagrangian admits right-handed vector and left- or right-handed tensor couplings as well. We find that the data prefer the left-handed vector coupling and set upper limits on the anomalous couplings. These are the first direct constraints on a general $Wtb$ interaction and the first direct limits on left- and right-handed tensor couplings.

Figure 1

Charged lepton $p_T$ spectrum from data and expectation for SM single top production plus background for events with two jets and one $b$-tagged jet. Superimposed are the distributions from single top quark production with different couplings (all other couplings set to zero) normalized to 10 times the SM single top quark cross section.

Figure 2

Boosted decision tree output distributions for data and sum of the SM signal and backgrounds for events with two jets and one $b$-tagged jet for (a) the $(L_1,L_2)$ scenario, (b) the $(L_1,R_1)$ scenario, and (c) the $(L_1,R_2)$ scenario. Superimposed are the distributions for the single top quark signals with different couplings normalized to 5 times the SM single top quark cross section.

Figure 3

Plots of the two-dimensional posterior probability density for the anomalous couplings. The plots on the left show the expectation for left-handed vector couplings, and the plots on the right show the observed posterior from our data. The upper row (a),(b) shows the plots for the $(L_1,L_2)$ scenario, the middle row (c),(d) for the $(L_1,R_1)$ scenario, and the bottom row (e),(f) for the $(L_1,R_2)$ scenario.