We calculate form-factor ratios between the semileptonic decays B⎯⎯⎯0→D+ℓ−ν⎯⎯ and B⎯⎯⎯0s→D+sℓ−ν⎯⎯ with lattice QCD. These ratios are a key theoretical input in a new strategy to determine the fragmentation fractions of neutral B decays, which are needed for measurements of BR(B0s→μ+μ−). We use the MILC ensembles of gauge configurations with 2+1 flavors of sea-quarks at two s of approximately 0.12 fm and 0.09 fm. We use the model-independent z parametrization to extrapolate our simulation results at small recoil toward maximum recoil. Our results for the form-factor ratios are f(s)0(M2π)/f(d)0(M2K)=1.046(44)stat(15)syst and f(s)0(M2π)/f(d)0(M2π)=1.054(47)stat(17)syst. In contrast to a QCD sum-rule calculation, no significant departure from U-spin (d↔s) symmetry is observed
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