First Measurement of σ(gg → t$\bar{t}$)/σ(p$\bar{p}$ → t$\bar{t}$)

Abstract

The work presented here is the first measurement of the fraction of top quark pair production through gluon-gluon fusion. We use an integrated luminosity of 0.96 ± 0.06 fb-1 of p{bar p} collisions at √s of 1.96 TeV collected by the CDF II detector. We select t$\bar{t}$ candidates by identifying a high-pT lepton candidate, a large missing ET as evidence for a neutrino candidate and at least four high ET jets, one of which has to be identified as originating from a b quark. The challenge is to discriminate between the two production processes with the identical final state, gg → t$\bar{t}$ and q$\bar{p}$ → t$\bar{t}$. We take advantage of the fact that compared to a quark, a gluon is more likely to radiate a low momentum gluon and therefore, one expects a larger number of charged particles with low pT in a process involving more gluons. Given the large uncertainties associated with the modeling of the low pT charged particle multiplicity, a data-driven technique was employed. Using calibration data samples, we show there exists a clear correlation between the observed average number of low pT charged particles and the average number of gluons involved in the production process predicted by Monte Carlo calculations. Given the correlation, one can identify low pT charged particle multiplicity distributions associated with specific average number of gluons. The W + 0 jet sample and dijets sample with leading jet ET in the range of 80-100 GeV are used to find no-gluon and gluon-rich low p{sub T} charged particle multiplicity distributions, respectively. Using these no-gluon and gluon-rich distributions in a likelihood fit, we find the fraction of gluon-rich events in t{bar t} candidates. This fraction has contributions from the signal and background events. Taking into account these contributions and the gg → t$\bar{t}$ and q$\bar{q}$ → t$\bar{t}$ production channel acceptances, we find σ(gg → t$\bar{t}$)/σ(p$\bar{p}$ → t$\bar{t}$) = 0.07 ± 0.14(stat) ± 0.07(syst) in agreement with the theoretical predictions of 0.15 ± 0.05. This measurement is a stepping stone towards a better understanding of the production mechanism of top quark pairs and implies that at least 67% of the t$\bar{t}$ events are consistent with q{bar q} production at 95% C.L, leaving little room for non SM processes which are not similar to the q$\bar{q}$ → t$\bar{t}$ in their gluon radiation. The result confirms our current understanding of the SM high pT production mechanism and the relative gluon PDFs at relatively high Q2 and x.