Measurement of D0-(D)over-bar0 mixing using the ratio of lifetimes for the decays D0 → K-π+ and K+K-

We measure the rate of D-0-(D) over bar0 mixing with the observable y(CP) = (tau(K pi)/tau(KK)) - 1, where tau(KK) and tau(K pi) are, respectively, the mean lifetimes of CP-even D-0 -> K+K- and CP-mixed D-0 -> K-pi(+) decays, using a data sample of 384 fb(-1) collected by the BABAR detector at the SLAC PEP-II asymmetric-energy B Factory. From a sample of D-0 and (D) over bar0 decays where the initial flavor of the decaying meson is not determined, we obtain y(CP) = [1.12 +/- 0.26(stat) +/- 0.22(syst)]%, 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 y(CP) obtained from a sample of D*(+) -> D-0 pi(+) events, where the D-0 rho decays to K-pi(+), K+K-, and pi(+)pi(-), which is disjoint with the untagged D-0 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 y(CP) = [1.16 +/- 0.22(stat) +/- 0.18(syst)]%, which excludes the no-mixing hypothesis at 4.1 sigma.