Search for Production of Single Top Quarks Via $tcg$ and $tug$ Flavor-Changing-Neutral-Current Couplings

We search for the production of single top quarks via flavor-changing-neutral-current couplings of a gluon to the top quark and a charm ($c$) or up ($u$) quark. We analyze $230{\mathrm{pb}}^{-1}$ of $\mathrm{\text{lepton}}+\mathrm{\text{jets}}$ data from $p\overline{p}$ collisions at a center of mass energy of 1.96 TeV collected by the D0 detector at the Fermilab Tevatron Collider. We observe no significant deviation from standard model predictions, and hence set upper limits on the anomalous coupling parameters ${\kappa }_g^c/\Lambda$ and ${\kappa }_g^u/\Lambda$, where ${\kappa }_g$ define the strength of $tcg$ and $tug$ couplings, and $\Lambda$ defines the scale of new physics. The limits at 95% C.L. are ${\kappa }_g^c/\Lambda <0.15\mathrm{Te}{\mathrm{V}}^{-1}$ and ${\kappa }_g^u/\Lambda <0.037{\mathrm{TeV}}^{-1}$.

Figure 1

Representative $2\to 2$ Feynman diagrams for single top quark production through flavor-changing neutral currents involving the gluon.

Figure 2

Distributions of (a) an input variable to neural networks, and outputs normalized to (b) unity, and (c) $230{\mathrm{pb}}^{-1}$ of data with backgrounds summed. The FCNC signal is for the summed $tcg$ and $tug$ processes, each evaluated at ${\kappa }_g/\Lambda =0.03{\mathrm{TeV}}^{-1}$.

Figure 3

Exclusion contours at various levels of confidence using $230{\mathrm{pb}}^{-1}$ of D0 data in both the electron and muon channels.