On the model dependence of measured Bs-meson branching fractions

TL;DR

Measuring $B_s$-meson branching fractions involves a significant model dependence due to lifetime differences, which can impact the interpretation of experimental results and constraints on new physics.

Contribution

The paper highlights the importance of explicitly accounting for model dependence in $B_s$-meson branching fraction measurements and proposes a two-dimensional presentation method.

Findings

Model dependence can be close to 10% in real-life scenarios.

Ignoring this dependence can lead to over-constraining new physics couplings.

The effect impacts upper limits on forbidden $B_s$ decay modes.

Abstract

The measurement of $B_s$-meson branching fractions is a fundamental tool to probe physics beyond the Standard Model. Every measurement of untagged time-integrated $B_s$-meson branching fractions is model-dependent due to the time dependence of the experimental efficiency and the large lifetime difference between the two $B_s$ mass eigenstates. In recent measurements, this effect is bundled in the systematics. We reappraise the potential numerical impact of this effect -- we find it to be close to 10% in real-life examples where new physics is a correction to dominantly Standard-Model dynamics. We therefore suggest that this model dependence be made explicit, i.e. that $B_s$ branching-fraction measurements be presented in a two-dimensional plane with the parameter that encodes the model dependence. We show that ignoring this effect can lead to over-constraining the couplings of new-physics models. In particular, we note that the effect also applies when setting upper limits on non-observed $B_s$ decay modes, such as those forbidden within the Standard Model.