We report improved measurements of $B$ decays with an $\eta$ meson in the final state using $492{\mathrm{fb}}^{-1}$ of data collected by the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. We observe the decays $B^{\pm }\to \eta {\pi }^{\pm }$ and $B^{\pm }\to \eta K^{\pm }$ and measure the branching fractions $\mathcal{B}(B^{\pm }\to \eta {\pi }^{\pm })=\mathbf{(}4.2\pm 0.4(\mathrm{stat})\pm 0.2(\mathrm{sys})\mathbf{)}\times {10}^{-6}$ and $\mathcal{B}(B^{\pm }\to \eta K^{\pm })=\mathbf{(}1.9\pm 0.3(\mathrm{stat}{)}_{-0.1}^{+0.2}(\mathrm{sys})\mathbf{)}\times {10}^{-6}$. The corresponding $CP$-violating asymmetries are measured to be $-0.23\pm 0.09(\mathrm{stat})\pm 0.02(\mathrm{sys})$ for $\eta {\pi }^{\pm }$ and $-0.39\pm 0.16(\mathrm{stat})\pm 0.03(\mathrm{sys})$ for $\eta K^{\pm }$. We also search for $B^0\to \eta K^0$ decays and set an upper limit of $1.9\times {10}^{-6}$ at the 90% confidence level.

• Received 9 February 2007

DOI:https://doi.org/10.1103/PhysRevD.75.071104