Eur. Phys. J. C (2025) 85: 995
https://doi.org/10.1140/epjc/s10052-025-14696-8

Regular Article - Theoretical Physics

Precision tests in $\mathit{b}\mathbf{\to }\mathit{s}{\mathit{\ell }}^{\mathbf{+}}{\mathit{\ell }}^{\mathbf{-}}$ ($\mathit{\ell }\mathbf{=}\mathit{e}\mathbf{,}\mathit{\mu }$) at FCC-ee

¹ Physik-Institut, Universität Zürich, 8057, Zürich, Switzerland
² Theoretical Physics Department, CERN, 1211, Geneva 23, Switzerland

^a claudia.cornella@cern.ch

Received: 11 April 2025
Accepted: 29 August 2025
Published online: 15 September 2025

Abstract

The rare semi-leptonic decays $B\to K^{\ast }{\ell }^{+}{\ell }^{-}$, with $\ell =e,\mu$, are highly sensitive to new physics (NP) due to their suppression in the Standard Model (SM). Current LHCb measurements in the muon channel exhibit a significant tension with state-of-the-art SM theory predictions. The proposed tera-Z run at FCC-ee provides a unique opportunity to untangle the origin of this tension by producing a very large sample of B-mesons in a clean $e^{+}{e}^{-}$ environment. We explore the expected precision of $B\to K^{\ast }{\ell }^{+}{\ell }^{-}$ ($\ell =e,\mu$) measurements at FCC-ee, complementing existing studies with ${\tau }^{+}{\tau }^{-}$ in the final state, and compare with HL-LHC projections. For the case of muons in the final state, we show that HL-LHC and FCC-ee are expected to deliver a similar number of events, while the latter performs much better in the case of final state electrons. Regardless of the lepton flavour, we expect the FCC-ee environment to be much cleaner than at HL-LHC, with subleading systematics. We also find that a significant reduction in theory uncertainties on the SM predictions is required to capitalize on the advantage going from HL-LHC to FCC-ee. We demonstrate the power of such measurements at FCC-ee to extract information on the long-distance contribution to these decays, and to reveal evidence for new physics even if no deviations are seen in electroweak precision tests.