$\bar B\to D\tau\bar \nu_\tau$ vs. $\bar B\to D\mu\bar \nu_\mu$

TL;DR

This paper examines the ratio of branching fractions in B meson decays to D mesons with tau and muon neutrinos, showing consistency with the Standard Model and potential for constraining New Physics.

Contribution

It demonstrates that the measured ratio aligns with the Standard Model within 2σ and proposes a minimal theoretical approach for experimental testing and New Physics constraints.

Findings

Ratio consistent with Standard Model within 2σ

Minimal theory input suffices for experimental tests

Decay channels can constrain New Physics couplings

Abstract

Recent experimental results for the ratio of the branching fractions of $\bar B\to D^{(*)}\tau\bar \nu_\tau$ and $\bar B\to D^{(*)}\mu\bar \nu_\mu$ decays came as a surprise and lead to a discussion of possibility to constraining New Physics through these modes. Here we focus on ${\cal B}(\bar B\to D\tau\bar \nu_\tau)/{\cal B}(\bar B\to D\mu\bar \nu_\mu)$ and argue that the result is consistent with the Standard Model within $2\sigma$, and that the test of compatibility of this ratio with the Standard Model can be done experimentally with a minimal theory input. We also show that these two decay channels can provide us with quite good constraints of the New Physics couplings.