Eur. Phys. J. C (2025) 85: 893
https://doi.org/10.1140/epjc/s10052-025-14603-1

Regular Article - Experimental Physics

Measuring $A_{\text{FB}}^b$ and $R_b$ with exclusive b-hadron decays at the FCC-ee

¹ Department of Physics, TU Dortmund University, Dortmund, Germany
² Université Clermont-Auvergne, CNRS, LPCA, 63000, Clermont-Ferrand, France
³ INFN Pisa, Pisa, Italy
⁴ CERN, EP Department, Geneva, Switzerland

^a lars.roehrig@tu-dortmund.de

Received: 20 March 2025
Accepted: 31 July 2025
Published online: 22 August 2025

Abstract

This paper presents a novel tagging technique to measure the beauty-quark partial decay-width ratio $R_b$ and its forward–backward asymmetry $A_{\text{FB}}^b$ at the FCC-ee, using $\mathcal{O}({10}^{12})$Z-boson decays. The method is based on the exclusive reconstruction of a selected list of b-hadron decay modes in $Z\to b\overline{b}$ events at the Z pole, which can provide the flavour and possibly the charge of the hemisphere. This approach effectively eliminates the contamination from light-quark physics events and reduces the leading systematic uncertainties arising from background contamination, tagging-efficiency correlations, and gluon-radiation corrections by exploiting the geometric and kinematic properties of beauty hadrons. This results in a total relative uncertainty of the order of 0.01 % for both observables. Furthermore, this precision allows to obtain a commensurate precision on the weak mixing angle ${sin}^2({\theta }_{\text{W}}^{\text{eff}})$ compared to the muon forward–backward asymmetry on the order of 0.002 %.