We report a measurement of the $CP$-violation parameter $\sin 2{\varphi }_1$ at the ${\rm Y}(5S)$ resonance using a new tagging method, called “$B\mathrm{\text{−}}\pi$ tagging.” In ${\rm Y}(5S)$ decays containing a neutral $B$ meson, a charged $B$, and a charged pion, the neutral $B$ is reconstructed in the $J/\psi K_S^0$ $CP$-eigenstate decay channel. The initial flavor of the neutral $B$ meson at the moment of the ${\rm Y}(5S)$ decay is opposite to that of the charged $B$ and may thus be inferred from the charge of the pion without reconstructing the charged $B$. From the asymmetry between $B\mathrm{\text{−}}{\pi }^{+}$ and $B\mathrm{\text{−}}{\pi }^{-}$ tagged $J/\psi K_S^0$ yields, we determine $\sin 2{\varphi }_1=0.57\pm 0.58(\mathrm{stat})\pm 0.06(\mathrm{syst})$. The results are based on $121{\mathrm{fb}}^{-1}$ of data recorded by the Belle detector at the KEKB $e^{+}{e}^{-}$ collider.

• Received 17 January 2012

DOI:https://doi.org/10.1103/PhysRevLett.108.171801