Measurement of the Ratio ${\sigma }_{t\overline{t}}/{\sigma }_{Z/{\gamma }^{*}\to ll}$ and Precise Extraction of the $t\overline{t}$ Cross Section

We report a measurement of the ratio of the $t\overline{t}$ to $Z/{\gamma }^{*}$ production cross sections in $\sqrt{s}=1.96\mathrm{TeV}$ $p\overline{p}$ collisions using data corresponding to an integrated luminosity of up to $4.6{\mathrm{fb}}^{-1}$, collected by the CDF II detector. The $t\overline{t}$ cross section ratio is measured using two complementary methods, a $b$-jet tagging measurement and a topological approach. By multiplying the ratios by the well-known theoretical $Z/{\gamma }^{*}\to ll$ cross section predicted by the standard model, the extracted $t\overline{t}$ cross sections are effectively insensitive to the uncertainty on luminosity. A best linear unbiased estimate is used to combine both measurements with the result ${\sigma }_{t\overline{t}}=7.70\pm 0.52\mathrm{pb}$, for a top-quark mass of $172.5\mathrm{GeV}/c^2$.

Figure 1

Number of data and predicted background events as a function of jet multiplicity, with the number of $t\overline{t}$ events normalized to the measured cross section. The hashed lines represent the uncertainty on the predicted number of events.

Figure 2

The output of an artificial neural network (ANN), trained to distinguish $t\overline{t}$ events from background, for simulated $t\overline{t}$ and background events, and data. The $t\overline{t}$ cross section is extracted from a fit of templates to the data.