Measurement of the Fraction of tt-bar Production via Gluon-Gluon Fusion in p anti-p Collisions at s**(1/2) =1.96 TeV

We present a measurement of the ratio of the tt-bar production cross section via gluon-gluon fusion to the total tt-bar production cross section in p anti-p collisions at s**(1/2) =1.96 TeV at the Tevatron. Using a data sample with an integrated luminosity of 955 pb-1 recorded by the CDF II detector at Fermilab, we select events based on the tt-bar decay to lepton+jets. Using an artificial neural network technique we discriminate between tt-bar events produced via qq-bar annihilation and gg fusion, and find G_f =sigma(gg --> tt-bar)/sigma(p anti-p --> tt-bar) < 0.33 at the 68% confidence level. This result is combined with a previous measurement to obtain the most stringent measurement of this quantity by CDF to date, G_f 0.070.15_-0.07.