Observation of the Decay B_{c}^{+}B_{s}^{0}pi^{+}

The result of a search for the decay B_{c}^{+}B_{s}^{0}pi^{+} is presented, using the B_{s}^{0}D_{s}^{-}pi^{+} and B_{s}^{0}J/psiϕ channels. The analysis is based on a data sample of pp collisions collected with the LHCb detector, corresponding to an integrated luminosity of 1fb^{-1} taken at a center-of-mass energy of 7TeV, and 2fb^{-1} taken at 8TeV. The decay B_{c}^{+}B_{s}^{0}pi^{+} is observed with significance in excess of 5 standard deviations independently in both decay channels. The measured product of the ratio of cross sections and branching fraction is [sigma(B_{c}^{+})/sigma(B_{s}^{0})]*B(B_{c}^{+}B_{s}^{0}pi^{+})=[2.370.31(stat)0.11(syst)_{-0.13}^{+0.17}(tau_{B_{c}^{+}})]*10^{-3}, in the pseudorapidity range 2<eta(B)<5, where the first uncertainty is statistical, the second is systematic, and the third is due to the uncertainty on the B_{c}^{+} lifetime. This is the first observation of a B meson decaying to another B meson via the weak interaction.