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

TL;DR

This paper introduces a new exclusive decay-based tagging method to precisely measure $A_{FB}^b$ and $R_b$ at the FCC-ee, significantly reducing systematic uncertainties and enabling high-precision electroweak measurements.

Contribution

It proposes a novel exclusive reconstruction technique for $b$-hadron decays at the FCC-ee, improving measurement precision of $A_{FB}^b$ and $R_b$ with reduced systematic uncertainties.

Findings

Achieves a total relative uncertainty of about 0.01% for $A_{FB}^b$ and $R_b$

Enables a precision on $ ext{sin}^2( heta_W^{ ext{eff}})$ of approximately 0.002%

Reduces contamination and systematic errors compared to previous methods

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 $0.002\,\%$.