We report the first lattice QCD calculation of the form factors for the standard model tree-level decay B_s→ K ℓν. In combination with future measurement, this calculation will provide an alternative exclusive semileptonic determination of |V_ub|. We compare our results with previous model calculations, make predictions for differential decay rates and branching fractions, and predict the ratio of differential branching fractions between B_s→ Kτν and B_s→ Kμν. We also present standard model

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... ictions for differential decay rate forward-backward asymmetries, polarization fractions, and calculate potentially useful ratios of B_s→ K form factors with those of the fictitious B_s→η_s decay. Our lattice simulations utilize NRQCD b and HISQ light quarks on a subset of the MILC Collaboration's 2+1 asqtad gauge configurations, including two lattice spacings and a range of light quark masses.