First observation of the decay B-c(+) -> J/psi K+

The decay B-c(+) -> J/psi 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 p p 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 B-c(+) -> J/psi K+ to that of B-c(+) -> J/psi pi(+) is measured to be 0.069 +/- 0.019 +/- 0 : 005, where the first uncertainty is statistical and the second is systematic.