University of Birmingham First Observation of the Decay Bs0 K-+ and a Measurement of |Vub | / |Vcb |

The first observation of the suppressed semileptonic B 0 s → K − μ þ ν μ decay is reported. Using a data sample recorded in pp collisions in 2012 with the LHCb detector, corresponding to an integrated luminosity of 2 fb − 1 , the branching fraction B ð B 0 s → K − μ þ ν μ Þ is measured to be ½ 1 . 06 (cid:2) 0 . 05 ð stat Þ(cid:2) 0 . 08 ð syst Þ(cid:3) × 10 − 4 , where the first uncertainty is statistical and the second one represents the combined systematic uncertainties. The decay B 0 s → D − s μ þ ν μ , where D − s is reconstructed in the final state K þ K − π − , is used as a normalization channel to minimize the experimental systematic uncertainty. Theoretical calculations on the form factors of the B 0 s → K − and B 0 s → D − s transitions are employed to determine the ratio of the Cabibbo-Kobayashi-Maskawa matrix elements j V ub j = j V cb j at low and high B 0 s → K − momentum transfer.