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

We report preliminary results on time-dependent CP asymmetries in $B \to D^{*\mp}\pi^{\pm}$ decays. The CP asymmetry in these decays is proportional to $2R_{D^{*} \pi} \sin (2\phi_1 + \phi_3 \pm \delta_{D^{*} \pi})$, where $R_{D^{*} \pi}$ is the ratio of the magnitudes of the doubly-Cabibbo-suppressed and Cabibbo-favoured amplitudes, $\delta_{D^{*} \pi}$ is the strong phase difference between them, and $\phi_1$ and $\phi_3$ 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 $\Upsilon(4S)$ resonance. We use a partial reconstruction technique, wherein signal $B \to D^{*\mp}\pi^{\pm}$ 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 $D^{*-}$. We obtain $S^+ (D^* \pi) = +0.057 \pm 0.019(\mathrm{stat}) \pm 0.012(\mathrm{sys})$ and $S^- (D^* \pi) = +0.038 \pm 0.020(\mathrm{stat}) \pm 0.010(\mathrm{sys})$.