Measurement of D0-D0bar Mixing using the Ratio of Lifetimes for the Decays D0->K-pi+ and K+K-

We measure the rate of D0-D0bar mixing with the observable yCP=(tauKpi/tauKK)-1, where tauKK and tauKpi are respectively the mean lifetimes of CP-even D0->K+K- and CP-mixed D0->K-pi+ decays, using a data sample of 384/fb collected by the Babar detector at the SLAC PEP-II asymmetric-energy B Factory. From a sample of D0 and D0bar decays where the inital flavor of the decaying meson is not determined, we obtain yCP = [1.12 +/- 0.26 (stat) +/- 0.22 (sys)]%, which excludes the no-mixing hypothesis at 3.3 sigma, including both statistical and systematic uncertainties. This result is in good agreement with a previous Babar measurement of yCP obtained from a sample of D*+->D0pi+ events, where the D0 decays to K-pi+, K+K-, and pi+pi-, which is disjoint with the untagged D0 events used here. Combining the two results taking into account statistical and systematic uncertainties, where the systematic uncertainties are assumed to be 100% correlated, we find yCP = [1.16 +/- 0.22 (stat) +/- 0.18 (sys)]%, which excludes the no-mixing hypothesis at 4.1 sigma.