The decay B+
c → 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
B+
c → J/ψ K+ to that of B+
c → J/ψ π+ is measured to be 0.069 ± 0.019 ± 0.005, where the
first uncertainty is statistical and the second is systematic.