Charmless charged two-body $B$ decays at LHCb with 2011 data

Using an integrated luminosity of 320 $\mathrm{pb}^{-1}$ collected by LHCb in the first part of the 2011 run, we reconstruct a large sample of the charmless charged two-body decay modes of $B$ hadrons, providing measurements of the direct $CP$ asymmetries $A_{CP}(B^0 \rightarrow K\pi)=-0.088 \pm 0.011\mathrm{(stat)} \pm 0.008\mathrm{(syst)}$ and $A_{CP}(B^0_s \rightarrow \pi K)=0.27 \pm 0.08\mathrm{(stat)} \pm 0.02\mathrm{(syst)}$. The former constitutes the best measurement in the world as well as the first observation of $CP$ violation at LHCb, while the latter is the first evidence of $CP$ violation in the $B^0_s \rightarrow \pi K$ decay. Furthermore, we measure the branching fractions $\mathcal{BR}(B^0 \rightarrow K^+K^-)=(0.13 ^{+0.06}_{-0.05} \mathrm{(stat)} \pm 0.07\mathrm{(syst)})\times10^{-6}$ and $\mathcal{BR}(B^0_s \rightarrow \pi^+\pi^-)=(0.98 ^{+0.23}_{-0.19} \mathrm{(stat)} \pm 0.11\mathrm{(syst)})\times10^{-6}$. In particular, the absence of a $B^0_s \rightarrow \pi^+\pi^- $ signal is rejected with a statistical significance of 5.3$\sigma$.