Observation of the decay $B_c^+ \to B_s^0 \pi^+$

The result of a search for the decay $B_c^+ \to B_s^0 \pi^+$ is presented, using the $B_s^0 \to D_s^- \pi^+$ and $B_s^0 \to J/\psi \phi$ channels. The analysis is based on a data sample of pp collisions collected with the LHCb detector, corresponding to an integrated luminosity of 1 fb$^{-1}$ taken at a center-of-mass energy of 7 TeV, and 2 fb$^{-1}$ taken at 8 TeV. The decay $B_c^+ \to B_s^0 \pi^+$ is observed with significance in excess of five standard deviations independently in both decay channels. The measured product of the ratio of cross-sections and branching fraction is $\sigma(B_c^+)/\sigma(B_s^0) \times \mathcal{B}(B_c^+ \to B_s^0 \pi^+) = (2.37 \pm 0.31 (\text{stat}) \pm 0.11 (\text{syst}) ^{+0.17}_{-0.13} (\tau_{B_c^+})) \times 10^{-3}$ 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.