First observation of the decay $ B_c^{+}\to {J \left/ {{\psi {K^{+}}}} \right.}

The decay $ B_c^{+}\to {J \left/ {{\psi {K^{+}}} \right.} $ 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^{+}\to {J \left/ {{\psi {K^{+}}} \right.} $ to that of $ B_c^{+}\to {J \left/ {{\psi {\pi^{+}}} \right.} $ is measured to be 0.069 ± 0.019 ± 0.005, where the first uncertainty is statistical and the second is systematic.