Remark on the theoretical uncertainty in B0–B 0 mixing

We re-examine the theoretical uncertainty in the Standard Model expression for B0–B 0 mixing. We focus on lattice calculations of the ratio ξ, needed to relate the oscillation frequency of B0s–B 0s mixing to the poorly known CKM element Vtd. We replace the usual linear chiral extrapolation with one that includes the logarithm that appears in chiral perturbation theory. We find a significant shift in the ratio ξ, from the conventional 1.15±0.05 to ξ=1.32±0.10.