Evidence for tt-bar gamma Production and Measurement of sigma_tt- bar gamma/sigma_tt-bar

Using data corresponding to 6.0  fb-1 of pp̅ collisions at √s=1.96  TeV collected by the CDF II detector, we present a cross section measurement of top-quark pair production with an additional radiated photon in the central region with 10 GeV or more of transverse energy tt̅ γ. The events are selected by looking for a lepton (ℓ or μ), a photon (γ), significant transverse momentum imbalance (E̸T), large total transverse energy, and three or more jets, with at least one identified as containing a b quark (b). Using an event selection optimized for the tt̅ γ candidate sample, we also measure the cross section of tt̅ (σtt̅ ). We measure the tt̅ γ cross section (σtt̅ γ) to be 0.18±0.08  pb, and the ratio of σtt̅ γ to σtt̅ to be 0.024±0.009. We observe a probability of 0.0015 (3.0 standard deviations) of the background (non-tt̅ γ events alone producing 30 events or more.