Soft-Photon Corrections to B̅→Dτ-v̅τ Relative to B̅→Dμ-v̅μ.

We evaluate long-distance electromagnetic (QED) contributions to B̅0→D+τ-v̅τ and B-→D0τ-v̅τ relative to B̅0→D+μ-v̅μ and B-→D0μ-v̅μ, respectively, in the standard model. We point out that the QED corrections to the ratios R(D+) and R(D0) are not negligible, contrary to the expectation that radiative corrections are almost canceled out in the ratio of the two branching fractions. The reason is that long-distance QED corrections depend on the masses and relative velocities of the daughter particles. We find that theoretical predictions for R(D+)τ/μ and R(D0)τ/μ can be amplified by ~4% and ~3%, respectively, for the soft-photon energy cut in the range 20-40 MeV. [ABSTRACT FROM AUTHOR]