Evidence of time-dependent $CP$ violation in the decay $B^0\to D^{*+}{D}^{*-}$

We report a measurement of the $CP$-odd fraction and the time-dependent $CP$ violation in $B^0\to D^{*+}{D}^{*-}$ decays, using $657\times {10}^6$ $B\overline{B}$ events collected at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We measure a $CP$-odd fraction of $R_{\perp }=0.125\pm 0.043(\mathrm{stat})\pm 0.023(\mathrm{syst})$. From the distributions of the proper-time intervals between a $B^0\to D^{*+}{D}^{*-}$ decay and the other $B$ meson in the event, we obtain evidence of $CP$ violation with measured parameters ${\mathcal{A}}_{D^{*+}{D}^{*-}}=0.15\pm 0.13(\mathrm{stat})\pm 0.04(\mathrm{syst})$ and ${\mathcal{S}}_{D^{*+}{D}^{*-}}=-0.96\pm 0.25(\mathrm{stat}{)}_{-0.16}^{+0.13}(\mathrm{syst})$.

Figure 1

(a) The $M_{\mathrm{bc}}$ distribution for $|\Delta E|<0.04\mathrm{GeV}$. (b) The $\Delta E$ distribution for $M_{\mathrm{bc}}>5.27\mathrm{GeV}/c^2$. The solid curve shows the result of the fit while the dotted curve is the background contribution.

Figure 2

The $\cos {\theta }_{\mathrm{tr}}$ distribution for events in the small signal region, the points with error bars represent data. The solid curve is the result of the fit, the dotted curve shows the background contribution. The $CP$-even and $CP$-odd contributions are the dotted-dashed and dashed curves, respectively, and are visible above the dotted background curve.

Figure 3

Top: $\Delta t$ distribution of well-tagged $B^0\to D^{*+}{D}^{*-}$ candidates ($r>0.5$) for $q=+1$ and $q=-1$. The shaded area is the background contribution while the solid and dashed curves are the superposition of the total PDFs for well-tagged $q=-1$ and $q=+1$ events, respectively. Bottom: fitted raw asymmetry of the top two distributions.