First Observation of $CP$ Violation in ${\overline{B}}^0\to D_{CP}^{(*)}{h}^0$ Decays by a Combined Time-Dependent Analysis of BABAR and Belle Data

We report a measurement of the time-dependent $CP$ asymmetry of ${\overline{B}}^0\to D_{CP}^{(*)}{h}^0$ decays, where the light neutral hadron $h^0$ is a ${\pi }^0$, $\eta$, or $\omega$ meson, and the neutral $D$ meson is reconstructed in the $CP$ eigenstates $K^{+}{K}^{-}$, $K_S^0{\pi }^0$, or $K_S^0\omega$. The measurement is performed combining the final data samples collected at the $\mathrm{\Upsilon }(4S)$ resonance by the BABAR and Belle experiments at the asymmetric-energy $B$ factories PEP-II at SLAC and KEKB at KEK, respectively. The data samples contain $(471\pm 3)\times {10}^6\hspace{0.17em}B\overline{B}$ pairs recorded by the BABAR detector and $(772\pm 11)\times {10}^6\hspace{0.17em}B\overline{B}$ pairs recorded by the Belle detector. We measure the $CP$ asymmetry parameters $-{\eta }_f\mathcal{S}=+0.66\pm 0.10(\text{stat})\pm 0.06(\text{syst})$ and $\mathcal{C}=-0.02\pm 0.07(\text{stat})\pm 0.03(\text{syst})$. These results correspond to the first observation of $CP$ violation in ${\overline{B}}^0\to D_{CP}^{(*)}{h}^0$ decays. The hypothesis of no mixing-induced $CP$ violation is excluded in these decays at the level of 5.4 standard deviations.

Figure 1

The $M_{\text{bc}}$ distributions (data points with error bars) and fit projections (solid line) of ${\overline{B}}^0\to D_{CP}^{(*)}{h}^0$ decays for (a) BABAR and (b) Belle. The dashed (dotted) lines represent projections of the signal (background) fit components.

Figure 2

The proper time interval distributions (data points with error bars) for $B^0$ tags (red) and ${\overline{B}}^0$ tags (blue) and the $CP$ asymmetries of ${\overline{B}}^0\to D_{CP}^{(*)}{h}^0$ decays for (a)–(b) BABAR and (c)–(d) Belle for candidates associated with high-quality flavor tags. The solid lines show projections of the sum of signal and background components in the fit, while the hatched areas show only the background components.