First Measurement Of Sigma Gg Ttbar Sigma Ppbar Ttbar
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First Measurement Of Sigma Gg Ttbar Sigma Ppbar Ttbar
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language : en
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Release Date : 2008
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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-p{sub T} lepton candidate, a large missing E{sub T} as evidence for a neutrino candidate and at least four high E{sub T} 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 q} → 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 p{sub T} in a process involving more gluons. Given the large uncertainties associated with the modeling of the low p{sub T} 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 p{sub T} 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 p{sub T} charged particle multiplicity distributions associated with specific average number of gluons. The W + 0 jet sample and dijets sample with leading jet E{sub T} 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 [sigma](gg → t{bar t})/[sigma](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 p{sub T} production mechanism and the relative gluon PDFs at relatively high Q2 and x.