The first observation of the $B^0_stoetaeta$ decay is reported. The study is based on a sample of proton-proton collisions corresponding to $3.0$ ${rm fb^{-1}}$ of integrated luminosity collected with the LHCb detector. The significance of the signal
is $6.4$ standard deviations. The branching fraction is measured to be $[3.31 pm 0.64,{rm (stat)} pm 0.28,{rm (syst)} pm 0.12,{rm (norm)}]times10^{-5}$, where the third uncertainty comes from the $B^{pm}toeta K^{pm}$ branching fraction that is used as a normalisation. In addition, the charge asymmetries of $B^{pm}toeta K^{pm}$ and $B^{pm}tophi K^{pm}$, which are control channels, are measured to be $(-0.2 pm1.3)%$ and $(+1.7pm1.3)%$, respectively. All results are consistent with theoretical expectations.
The CP-violating charge asymmetry in $B^{pm}rightarrow phi K^{pm}$ decays is measured in a sample of $pp$ collisions at 7 TeV centre-of-mass energy, corresponding to an integrated luminosity of 1.0 fb$^{-1}$ collected by the LHCb experiment. The resu
lt is $mathcal{A}_{CP}(B^{pm}rightarrow phi K^{pm}) = rm 0.022pm 0.021 pm 0.009$, where the first uncertainty is statistical and the second systematic. In addition, a search for the $B^{pm}rightarrow phi pi^{pm}$ decay mode is performed, using the $B^{pm}rightarrow phi K^{pm}$ decay rate for normalization. An upper limit on the branching fraction $mathcal{B}(B^{pm}rightarrow phi pi^{pm})< 1.5times 10^{-7}$ is set at 90% confidence level.
