Soft-Photon Corrections to $\bar{B} \to D \tau^{-} \bar{\nu}_{\tau}$ Relative to $\bar{B} \to D \mu^{-} \bar{\nu}_{\mu}$

TL;DR

This paper calculates long-distance electromagnetic corrections to B meson decays involving tau and muon neutrinos, revealing non-negligible effects on the ratios R(D) due to mass and velocity dependencies of QED corrections.

Contribution

It provides the first detailed analysis of long-distance QED effects on R(D) ratios, showing they can significantly alter theoretical predictions.

Findings

QED corrections to R(D) ratios are not negligible.

Long-distance QED effects depend on daughter particle masses and velocities.

Predicted R(D) ratios can increase by about 3-4% with soft-photon energy cuts.

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.