b → sτ+τ− physics at future Z factories

Abstract

Abstract b → sτ+τ− measurements are highly motivated for addressing lepton-flavor-universality (LFU)-violating puzzles such as $$ {R}_{K^{\left(\ast \right)}} $$ R K ∗ anomalies. The anomalies of $$ {R}_{D^{\left(\ast \right)}} $$ R D ∗ and RJ/ψ further strengthen their necessity and importance, given that the LFU-violating hints from both involve the third-generation leptons directly. Z factories at the future e−e+ colliders stand at a great position to conduct such measurements because of their relatively high production rates and reconstruction efficiencies for B mesons at the Z pole. To fully explore this potential, we pursue a dedicated sensitivity study in four b → sτ+τ− benchmark channels, namely B0 → K*0τ+τ−, Bs → ϕτ+τ−, B+ → K+τ+τ− and Bs → τ+τ−, at the future Z factories. We develop a fully tracker-based scheme for reconstructing the signal B mesons and introduce a semi-quantitative method for estimating their major backgrounds. The simulations indicate that branching ratios of the first three channels can be measured with a precision ∼ $$ \mathcal{O} $$ O (10−7− 10−6) and that of Bs → τ+τ− with a precision ∼ $$ \mathcal{O} $$ O (10−5) at Tera-Z. The impacts of luminosity and tracker resolution on the expected sensitivities are explored. The interpretations of these results in effective field theory are also presented.