Evidence for $D^0 - \bar{D}^0$ Mixing

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