We report a first measurement of inclusive $B\to X_s\eta$ decays, where $X_s$ is a charmless state with unit strangeness. The measurement is based on a pseudoinclusive reconstruction technique and uses a sample of $657\times {10}^{6}B\overline{B}$ pairs accumulated with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. For $M_{X_s}<2.6\mathrm{GeV}/c^2$, we measure a branching fraction of $[26.1\pm 3.0(\mathrm{stat}{)}_{-2.1}^{+1.9}(\mathrm{syst}{)}_{-7.1}^{+4.0}(\mathrm{model})]\times {10}^{-5}$ and a direct $CP$ asymmetry of ${\mathcal{A}}_{CP}=-0.13\pm {0.04}_{-0.03}^{+0.02}$. Over half of the signal occurs in the range $M_{X_s}>1.8\mathrm{GeV}/c^2$.

• Received 19 August 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.191803