Abstract: We report a measurement of the ratio of the tt to Z= production cross sections in ffiffi s p ¼ 1:96 TeV pp collisions using data corresponding to an integrated luminosity of up to 4:6 fb1, collected by the CDF II detector. The tt cross section ratio is measured using two complementary methods, a b-jet tagging measurement and a topological approach. By multiplying the ratios by the well-known theoretical Z= ! ll cross section predicted by the standard model, the extracted tt cross sections are effectively insensitive to the uncertainty on luminosity. A best linear unbiased estimate is used to combine both measurements with the result tt ¼ 7:70 0:52 pb, for a top-quark mass of 172:5 GeV=c2.

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