Evidence forD0−D¯0Mixing

We observe evidence for $D^0\mathrm{\text{−}}{\overline{D}}^0$ mixing by measuring the difference in the apparent lifetime when a $D^0$ meson decays to the $CP$ eigenstates $K^{+}{K}^{-}$ and ${\pi }^{+}{\pi }^{-}$ and when it decays to the final state $K^{-}{\pi }^{+}$. We find the relative difference of the lifetimes $y_{CP}$ to be $[1.31\pm 0.32(\mathrm{stat})\pm 0.25(\mathrm{syst})]\%$, 3.2 standard deviations from zero. We also search for a $CP$ asymmetry between $D^0$ and ${\overline{D}}^0$ decays; no evidence for $CP$ violation is found. These results are based on $540{\mathrm{fb}}^{-1}$ of data recorded by the Belle detector at the KEKB $e^{+}{e}^{-}$ collider.