Measurement of D(0)-(D)over-bar(0) mixing using the ratio of lifetimes for the decays D(0) -> K(-)pi(+) 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.