First observation of the decay $B_s^0 \to K^-\mu^+\nu_\mu$ and Measurement of $|V_{ub}|/|V_{cb}|$

Abstract

The first observation of the suppressed semileptonic $B_s^0 \to K^-\mu^+\nu_\mu$ 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 $\mathcal{B}(B_s^0 \to K^-\mu^+\nu_\mu)$ is measured to be $[1.06±0.05(stat)±0.08(syst)]×10^{-4}$, where the first uncertainty is statistical and the second one represents the combined systematic uncertainties. The decay $B_s^0 → D_s^- μ^+ ν_μ$, where Ds- is reconstructed in the final state $K^+ K^- π^-$, is used as a normalization channel to minimize the experimental systematic uncertainty. Theoretical calculations on the form factors of the $B_s^0 → K^-$ and $B_s^0 → D_s^-$ transitions are employed to determine the ratio of the Cabibbo-Kobayashi-Maskawa matrix elements $|V_{ub}|/|V_{cb}|$ at low and high $B_s^0 → K^-$ momentum transfer.