Search for right-handed $W$ bosons in top quark decay

We present a measurement of the fraction $f_{+}$ of right-handed $W$ bosons produced in top quark decays, based on a candidate sample of $t\overline{t}$ events in the lepton+jets decay mode. These data correspond to an integrated luminosity of $230{\mathrm{pb}}^{-1}$, collected by the D0 detector at the Fermilab Tevatron $p\overline{p}$ Collider at $\sqrt{s}=1.96\mathrm{TeV}$. We use a constrained fit to reconstruct the kinematics of the $t\overline{t}$ and decay products, which allows for the measurement of the leptonic decay angle ${\theta }^{*}$ for each event. By comparing the $\cos {\theta }^{*}$ distribution from the data with those for the expected background and signal for various values of $f_{+}$, we find $f_{+}=0.00\pm 0.13(\mathrm{\text{stat}})\pm 0.07(\mathrm{\text{syst}})$. This measurement is consistent with the standard model prediction of $f_{+}=3.6\times {10}^{-4}$.

Figure 1

$\cos {\theta }^{*}$ distribution observed in the kinematic analysis. The standard model prediction is shown as the solid line, while a model with a pure $V+A$ interaction would result in the distribution given by the dashed line.

Figure 2

$\cos {\theta }^{*}$ distribution observed in the $b$-tagged analysis. The standard model prediction is shown as the solid line, while a model with a pure $V+A$ interaction would result in the distribution given by the dashed line.

Figure 3

$-\ln L$ curve obtained in the (a) kinematic analysis, (b) $b$-tagged analysis, and (c) kinematic and $b$-tagged analyses combined. The dashed line includes only the statistical uncertainty while the solid line also includes the systematic uncertainties. The physically allowed region for $f_{+}$ is indicated by the gray area.