Measurements of time-dependent CP Asymmetries in $B\to D^{\ast \mp }{\pi }^{\pm }$ decays using a partial reconstruction technique

We report preliminary results on time-dependent CP asymmetries in $B \to D^{\mp}\pi^{\pm}$decays\mathrm{.}TheCPasymmetryinthesedecaysisproportionalto$2R_{D^{} \pi} \sin (2\phi_1 + \phi_3 \pm \delta_{D^{} \pi})$,where$R_{D^{} \pi}$ is the ratio of the magnitudes of the doubly-Cabibbo-suppressed and Cabibbo-favoured amplitudes, ${\delta }_{D^{\ast }\pi }$ is the strong phase difference between them, and ${\varphi }_1$ and ${\varphi }_3$ are two angles of the CKM Unitarity Triangle. This study is based on a large data sample that contains 657 million $B\bar{B}$ pairs collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider at the $\mathrm{{\rm Y}}(4S)$ resonance. We use a partial reconstruction technique, wherein signal $B\to D^{\ast \mp }{\pi }^{\pm }$ events are identified using information only from the ${\pi }^{\pm }$ from the $B$ decay and the charged slow pion from the subsequent decay of the $D^{\ast -}$. We obtain $S^{+}(D^{\ast }\pi )=+0.057\pm 0.019(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.012(\mathrm{s}\mathrm{y}\mathrm{s})$ and $S^{-}(D^{\ast }\pi )=+0.038\pm 0.020(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.010(\mathrm{s}\mathrm{y}\mathrm{s})$.