First measurement of the fraction of top-quark pair production through gluon-gluon fusion

We present the first measurement of $\sigma (gg\to t\overline{t})/\sigma (p\overline{p}\to t\overline{t})$. We use $0.96{\mathrm{fb}}^{-1}$ of $\sqrt{s}=1.96\mathrm{TeV}$ $p\overline{p}$ collision data recorded with the CDF II detector at Fermilab. Using charged particles with low transverse momentum in $t\overline{t}$ events, we find $\sigma (gg\to t\overline{t})/\sigma (p\overline{p}\to t\overline{t})=0.07\pm 0.14(\mathrm{stat})\pm 0.07(\mathrm{syst})$, corresponding to a 95% confidence level upper limit of 0.33, in agreement with the standard model next-to-leading-order prediction of $0.15\pm 0.05$.

Figure 1

The correlation between the average number of low-$p_T$ charged particles (data) and the average number of gluons (MC). The dotted line is from a linear fit to the points.

Figure 2

Comparison between the normalized gluon-rich and no-gluon distributions and parametrizations.

Figure 3

The number of low-$p_T$ charged particles for the $t\overline{t}$ candidates, the fit result, the gluon-rich, and no-gluon components.