Quantify QED-modeling uncertainties for heavy-flavor Z-pole observables

Determine quantitatively the reduction in QED Monte Carlo modeling uncertainties for the heavy-flavor Z-pole observables R_b, R_c, A_FB^b, A_FB^c, and P_tau relative to the corresponding leptonic observables R_l and A_FB^l.

Background

The report estimates theory uncertainties from multi-photon radiation and fermion-pair creation using existing studies, but notes that dedicated estimates are unavailable for several heavy-flavor observables. It expects their uncertainties to be smaller than those for leptonic quantities because heavy-flavor event selections are largely inclusive with respect to final-state photon radiation.

A reliable quantitative reduction factor is needed to assign theory uncertainties to R_b, R_c, A_FBb, A_FBc, and P_tau and to incorporate them consistently into future electron–positron collider sensitivity projections.

References

At present, no dedicated uncertainty estimates are available for the observables $R_{b,c}$, $A_{\rm FB}{b,c}$, and $P_\tau$. For observables involving bottom and charm final states, the corresponding uncertainties are nevertheless expected to be smaller than those for the leptonic quantities $R_\ell$ and $A_{\rm FB}\ell$. This expectation is motivated by the fact that the event selection for heavy-flavor final states is largely inclusive with respect to photon radiation in the final state. However, in the absence of a quantitative assessment, the degree of this reduction cannot currently be determined reliably.

— Theoretical and phenomenological aspects of the Electroweak physics WG studies for the 2026 ESPPU Physics Briefing Book  (2609.18607 - Blas et al., 16 Sep 2026) in Section 2.1.1, subsection “Extraction of pseudo-observables,” paragraph on QED Monte Carlo modeling