Model-independent prediction of R(ηc)

We present a model-independent prediction for R(ηc)≡ℬℛ(Bc+ → ηcτ+ντ)/ℬℛ(Bc+ → ηcµ+νµ). This prediction is obtained from the form factors through a combination of dispersive relations, heavy-quark relations at zero-recoil, and the limited existing determinations from lattice QCD. The resulting prediction, R(ηc) = 0.29(5), agrees with previous model predictions, but without uncontrolled systematic uncertainties.