The result of a search for the decay Bc+ > Bs0 pi+ is presented, using the Bs0 > Ds pi+ and Bs0 > 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 fb1 taken at a centerofmass energy of 7 TeV, and 2 fb1 taken at 8 TeV. The decay Bc+ > Bs0 pi+ is observed with significance in excess of five standard deviations independently in both decay channels. The measured product of the ratio of crosssections and branching fraction is sigma(Bc+)/sigma(Bs0) x BR(Bc+ > Bs0 pi+) = (2.37 \pm 0.31 (stat) \pm 0.11 (syst)^{+0.17}_{0.13} (tau_Bc)) x 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 Bc lifetime. This is the first observation of a B meson decaying to another B meson via the weak interaction.
Leadingorder Feynman diagram of the decay $ B _ c ^+ \rightarrow B ^0_ s \pi ^+ $. 
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Invariant mass distributions of (a) $ B ^0_ s \rightarrow D ^_ s \pi ^+ $ and (b) $ B ^0_ s \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \phi$ candidates. The different components are defined in the legend. 
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$ B _ c ^+ $ mass fits for the combined 2011 and 2012 data sets for (a) $ B _ c ^+ \rightarrow B ^0_ s (\rightarrow D ^_ s \pi ^+ ) \pi ^+ $ and (b) $ B _ c ^+ \rightarrow B ^0_ s (\rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \phi) \pi ^+ $ candidates. The different components are indicated in the legends. 
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Animated gif made out of all figures. 
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Contributions of the various sources of (relative) systematic uncertainty on the efficiencycorrected ratio of event yields. The total systematic uncertainty is the quadratic sum of the individual contributions. The number of $ B _ c ^+ \rightarrow B ^0_ s (\rightarrow D ^_ s \pi ^+ ) \pi ^+ $ candidates is large enough that the peak position and width are freely varied in the fit, and hence the corresponding uncertainty is contained in the statistical uncertainty of the signal yield. 
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Created on 19 February 2021.