The first observation of the suppressed semileptonic $B_s^0 \to
K^-μ^+ν_μ$ decay is reported. Using a data sample recorded in {\it pp}
collisions in 2012 with the LHCb detector, corresponding to an integrated
luminosity of 2 $\mathrm{fb}^{-1}$, the branching fraction
\mbox{$\mathcal{B}(B_s^0 \to K^-μ^+ν_μ)$} is measured to be
$(1.06\pm0.05~(\mathrm{stat})\pm0.08~(\mathrm{syst}))\times 10^{-4}$, where the
first uncertainty is statistical and the second one represents the combined
systematic uncertainties. The decay $B_s^0 \to D_s^-μ^+ν_μ$, where
$D_s^-$ 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 $B_s^0 \to K^-$ and $B_s^0
\to D_s^-$ transitions are employed to determine the ratio of the CKM matrix
elements ${|V_{ub}|}/{|V_{cb}|}$ at low and high $B_s^0 \to K^-$ momentum
transfer.
Stephen Sekula
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