Soft-Photon Corrections to $\bar{B} \to D τ^{-} \barν_τ$ Relative to $\bar{B} \to D μ^{-} \barν_μ$

Abstract: We evaluate long-distance electromagnetic (QED) contributions to $\bar{B}{}^0 \to D^+ \tau^{-} \bar{\nu}{\tau}$ and $B^- \to D^0 \tau^{-} \bar{\nu}{\tau}$ relative to $\bar{B}{}^0 \to D^+ \mu^{-} \bar{\nu}{\mu}$ and $B^- \to D^0 \mu^{-} \bar{\nu}{\mu}$, respectively, in the standard model. We point out that the QED corrections to the ratios $R(D^{+})$ and $R(D^{0})$ 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 $R(D^{+})^{\tau/\mu}$ and $R(D^{0})^{\tau/\mu}$ can be amplified by $\sim4\%$ and $\sim3\%$, respectively, for the soft-photon energy cut in range $20$-$40$ MeV.