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A search for the $B^{0}\to K^{+}K^{-}$ decay is performed using$pp$-collision data collected by LHCb. The data set corresponds to integratedluminosities of 1 and 2 fb$^{-1}$ at center-of-mass energies of 7 and 8 TeV,respectively. This decay is observed for the first time, with a significance ofmore than five standard deviations. The analysis also results in an improvedmeasurement of the branching fraction for the $B_s^0\to \pi^+\pi^-$ decay. Themeasured branching fractions are $BR(B^0\to K^+K^-) = (7.80 \pm 1.27 \pm 0.81\pm 0.21) \times 10^{-8}$ and $BR(B_s^0\to\pi^+\pi^-) = (6.91 \pm 0.54 \pm 0.63\pm 0.19 \pm 0.40) \times 10^{-7}$. The first uncertainty is statistical, thesecond is systematic, the third is due to the uncertainty on the $B^0\toK^+\pi^-$ branching fraction used as a normalization. For the $B_s^0$ mode, thefourth accounts for the uncertainty on the ratio of the probabilities for $b$quarks to hadronize into $B_s^0$ and $B^0$ mesons.

Observation of the annihilation decay mode $B^{0}\to K^{+}K^{-}$