New determination of |V _ub /V _cb | from Bs0 → {K ⁻, Ds− }μ ⁺ ν

We update the full set of B¯ _s → K form factors using light-cone sum rules with an on-shell kaon. Our approach determines the relevant sum rule parameters — the duality thresholds — from a Bayesian fit for the first time. Using a modified version of the Boyd-Grinstein-Lebed parametrisation, we combine our sum rule results at low momentum transfer q ² with more precise lattice QCD results at large q ². We obtain a consistent description of the form factors in the full q ² range. Applying these results to a recent LHCb measurement of branching ratios for the decays Bs0 → {K ⁻, Ds− }μ ⁺ ν _μ, we determine the ratio of Cabibbo-Kobayashi-Maskawa elements |VubVcb|q2<7GeV2=0.0681±0.0040and|VubVcb|q2>7GeV2=0.0801±0.0047, which are mutually compatible at the 1.9σ level. We further comment on the sensitivity to Beyond the Standard Model effects through measurements of the shape of Bs0 → K ⁻ μ ⁺ ν _μ decays, in light of recent limits on such effects from other exclusive b → uℓν processes.