Evidence for D0 - D0(bar) Mixing at BELLE

The present paper reports on the evidence for D0 - D0(bar) mixing. We searched for mixing in two processes using 540 fb-1 of data recorded by the BELLE detector at the KEKB e+e- collider. In the first case, we measured the apparent lifetime for D0 meson decaying to CP eigenstates K+K- or π+π- and the lifetime in the D0 decays to K-π+. From the difference of the two lifetimes, the mixing parameter yCP is found to be yCP = (1.31 ± 0.32(stat.) ± 0.25(syst.))%, 3.2 standard deviations from zero. In the second measurement, we analyzed the time dependent Dalitz plot for the D0 → K0s π+π- decays, from which the mixing parameters x = (0.80 ± 0.29+0.09+0.15-0.07-0.14)% and y = (0.33 ± 0.24+0.07+0.08-0.12-0.09)% are determined; the errors are statistical, experimental systematic, and systematic due to the Dalitz decay model, respectively. We also searched for a CP asymmetry between D0 and D0(bar) decays, and found no evidence for it. (author)