Exclusion of exotic top-like quarks with $\mathbf{-}4/3$ electric charge using jet-charge tagging in single-lepton $t\overline{t}$ events at CDF

We report on a measurement of the top-quark electric charge in $t\overline{t}$ events in which one $W$ boson originating from the top-quark pair decays into leptons and the other into hadrons. The event sample was collected by the CDF II detector in $\sqrt{s}=1.96\mathrm{TeV}$ proton-antiproton collisions and corresponds to $5.6{\mathrm{fb}}^{-1}$. We find the data to be consistent with the standard model and exclude the existence of an exotic quark with $-4/3$ electric charge and mass of the conventional top quark at the 99% confidence level.

Figure 1

Examples of $p_{\mathrm{Trel}}$ (top panels) and secondary-vertex mass (bottom panels) templates for bottom, charm, and light quarks for different values of away-jet $E_T$.

Figure 2

Ratio between the purity of the JetQ algorithm in data and Monte Carlo simulation as a function of away-jet $E_T$. The dotted line is the result of fitting a constant through the data points.

Figure 3

Distribution of minimum ${\chi }^2$ for events passing selection requirements described in Sec. 3. Shaded histograms show signal and background predictions stacked to form the total prediction. The arrow shows the ${\chi }^2$ upper threshold.

Figure 4

Distribution of the number of tracks entering the JetQ calculation. Shaded histograms show signal and background predictions stacked to form the total prediction. The purity of the JetQ algorithm is calibrated as described in Sec. 6a, and a scale factor to the Monte Carlo is obtained to account for modeling discrepancies.

Figure 5

Lepton $p_T$ distribution. Shaded histograms show signal and background predictions stacked to form the total prediction.

Figure 6

Distribution of the maximum-likelihood estimate of the SM fraction ${\widehat{f}}_{+}$ from pseudoexperiments under the XM (dashed line) and the SM (solid line) hypotheses. The arrow shows our result.

Figure 7

Variation of ${\beta }_{\mathrm{XM}}$ (the probability of accepting a false XM) with ${\alpha }_{\mathrm{XM}}$ (the probability of rejecting a true XM). The square represents our a priori choice of ${\alpha }_{\mathrm{XM}}=1\%$, corresponding to ${\beta }_{\mathrm{XM}}=0.16\%$, while the triangle represents the observed $p$ values and is plotted at the coordinates $(p_{\mathrm{XM}},p_{\mathrm{SM}})$.

Figure 8

Distribution of the product of the $W$-boson charge times the JetQ value. Shaded histograms show signal and background predictions stacked for the total prediction. The dashed line shows the XM expectation. SM-like candidates are on the negative side of the plot while XM-like candidates are on the positive side. The outermost bins correspond to the cases where JetQ is exactly $\pm 1$.

Figure 9

Distribution of twice the negative logarithm of the profile likelihood as a function of the fraction of SM candidate events in data.

Figure 10

Distribution of the maximum-likelihood estimate of the SM fraction ${\widehat{f}}_{+}$ from pseudoexperiments under the XM (dashed line) and the SM (solid line) hypothesis for electrons only. The arrow shows our result.

Figure 11

Distribution of the maximum-likelihood estimate of the SM fraction ${\widehat{f}}_{+}$ from pseudoexperiments under the XM (dashed line) and the SM (solid line) hypothesis for muons only. The arrow shows our result.