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arXiv · 2502.17281

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

Lars Röhrig·Kevin Kröninger·Romain Madar·Stéphane Monteil·Fabrizio Palla·Emmanuel Perez
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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 $Z\to b\bar{b}$ 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 $\sin^2(\theta_W^\text{eff})$ compared to the muon forward-backward asymmetry on the order of $0.002\,\%$.

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Lars Röhrig, Kevin Kröninger, Romain Madar, Stéphane Monteil, Fabrizio Palla, Emmanuel Perez. 2025-02-24. Measuring $A_\text{FB}^b$ and $R_b$ with exclusive $b$-hadron decays at the FCC-ee. https://arxiv.org/abs/2502.17281

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