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

We report results on time-dependent $CP$ asymmetries in $B\to D^{*\mp }{\pi }^{\pm }$ decays based on a data sample containing $657\times {10}^6$ $B\overline{B}$ 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 $B\to D^{*\mp }{\pi }^{\pm }$ events are identified using information only from the fast pion from the $B$ decay and the slow pion from the subsequent decay of the $D^{*\mp }$, where the former (latter) corresponds to $D^{*+}(D^{*-})$ final states. We obtain $CP$ violation parameters $S^{+}=+0.061\pm 0.018(\mathrm{stat})\pm 0.012(\mathrm{syst})$ and $S^{-}=+0.031\pm 0.019(\mathrm{stat})\pm 0.015(\mathrm{syst})$.

Figure 1

Diagrams for $B^0\to D^{*-}{\pi }^{+}$ (left) and ${\overline{B}}^0\to D^{*-}{\pi }^{+}$ (right). Those for ${\overline{B}}^0\to D^{*+}{\pi }^{-}$ and $B^0\to D^{*+}{\pi }^{-}$ can be obtained by charge conjugation.

Figure 2

Results of the fits to $D^{*}\pi$ candidates projected onto $p_{\parallel }$ (left) and $p_{\delta }$ (right) in the signal region of the two kinematic variables. The contributions are $D^{*}\pi$ (open), $D^{*}\rho$ (green slanted lines), correlated background (magenta crossed lines), and uncorrelated background (shaded blue). Data are shown as points with error bars.

Figure 3

$p_{\perp }$ distributions for various MC simulations, showing the following contributions: $D^{*}\pi$ (red solid line), $D^{*}\rho$ (dotted green line), correlated background (dashed magenta line), and uncorrelated background (blue solid line).

Figure 4

$\Delta z$ distributions for four flavor-charge combinations: ${\pi }^{-}{l}^{-}$ (top left), ${\pi }^{-}{l}^{+}$ (top right), ${\pi }^{+}{l}^{-}$ (bottom left), and ${\pi }^{+}{l}^{+}$ (bottom right). The fit result (solid blue line) is superimposed on the data (solid points with error-bars). The signal and background components are shown as the solid red and dotted black curves, respectively.

Figure 5

Results of the fit to obtain $S^{+}$ and $S^{-}$, shown as asymmetries in the SF events (left) and OF events (right). The fit results (solid blue lines) are superimposed on the data.