First observation of the decay Bc+ → J/ψ K+

The decay Bc+ → J/ψ K+ is observed for the first time using a data sample, corresponding to an integrated luminosity of 1.0 fb-1, collected by the LHCb experiment in pp collisions at a centre-of-mass energy of 7 TeV. A yield of 46 ± 12 events is reported, with a significance of 5.0 standard deviations. The ratio of the branching fraction of Bc+→ J/ψ K+ to that of Bc+ → J/ψ pi+ is measured to be 0.069 ± 0.019 ± 0.005, where the first uncertainty is statistical and the second is systematic. © 2013 Cern for the benefit of the LHCb collaboration.