Observation of the decay B_c^+ \to 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/psi O 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)(+) 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+) = [2.37 ± 0.31 (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.