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

Abstract

We report preliminary results on time-dependent CP asymmetries in $B\to D^{\ast \mp }{\pi }^{\pm }$ decays. The CP asymmetry in these decays is proportional to $2R_{D^{\ast }\pi }\sin (2{\varphi }_1+{\varphi }_3\pm {\delta }_{D^{\ast }\pi })$, where $R_{D^{\ast }\pi }$ 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 ${\rm Y}(4S)$ 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 $S^{+}(D^{\ast }\pi )=+0.057\pm 0.019(\mathrm{stat})\pm 0.012(\mathrm{sys})$ and $S^{-}(D^{\ast }\pi )=+0.038\pm 0.020(\mathrm{stat})\pm 0.010(\mathrm{sys})$.

arXiv comment13 pages, 4 figures, Contributed to the 34th International Conference On High Energy Physics (ICHEP 2008)