Search for $CP$ Violation in the Decays $D^0\to K^{-}{K}^{+}$ and $D^0\to {\pi }^{-}{\pi }^{+}$

We measure time-integrated $CP$-violating asymmetries of neutral charmed mesons in the modes $D^0\to K^{-}{K}^{+}$ and $D^0\to {\pi }^{-}{\pi }^{+}$ with the highest precision to date by using $D^0\to K^{-}{\pi }^{+}$ decays to correct detector asymmetries. An analysis of $385.8{\mathrm{fb}}^{-1}$ of data collected with the BABAR detector yields values of $a_{CP}^{KK}=\mathbf{(}0.00\pm 0.34(\mathrm{stat})\pm 0.13(\mathrm{syst})\mathbf{)}\%$ and $a_{CP}^{\pi \pi }=\mathbf{(}-0.24\pm 0.52(\mathrm{stat})\pm 0.22(\mathrm{syst})\mathbf{)}\%$, which agree with standard model predictions.

Figure 1

Invariant mass distributions of the $KK$ final state tagged as (a) $D^0$ and (b) ${\overline{D}}^0$, and the $\pi \pi$ final state tagged as (c) $D^0$ and (d) ${\overline{D}}^0$. Distributions of data (points with error bars) in the signal region $0.1434<\Delta m<0.1474\mathrm{GeV}/c^2$ are overlaid with fitted PDFs (dashed line, shaded areas). The white regions under the central peaks represent signal events, the light gray misassociated ${\pi }_s^{\pm }$ events, and the dark gray remaining nonpeaking background. The data are shown over ranges extended beyond the fitted regions to illustrate the physical background shapes.

Figure 2

$K\pi$ efficiency-map components obtained from the nontagged $D^0$ daughters (a) $K$ and (b) $\pi$, and (c) ${\pi }_s$ efficiency map with (d) statistical errors from the tagged $K\pi$ sample. Maps are produced from the ratios of candidate numbers of $D^0$ to ${\overline{D}}^0$.

Figure 3

$CP$-violating asymmetries in (a) $KK$ and (b) $\pi \pi$, and forward-backward asymmetries in (c) $KK$ and (d) $\pi \pi$. In (a) and (b), the dashed lines represent the central values and the hatched regions the $1\sigma$ intervals, obtained from ${\chi }^2$ minimizations.