Lepton polarization asymmetries in rare semi-tauonic $b\to s$ exclusive decays at FCC-$ee$

Abstract

We consider measurements of exclusive rare semi-tauonic $b$-hadron decays, mediated by the $b\to s{\tau }^{+}{\tau }^{-}$ transition, at a future high energy circular electron-positron collider (FCC-$ee$). We argue that the high boosts of $b$-hadrons originating from on-shell $Z$ boson decays allow for a full reconstruction of the decay kinematics in hadronic $\tau$ decay modes (up to discrete ambiguities). This, together with the potentially large statistics of $Z\to b\bar{b}$, opens the door for the experimental determination of $\tau$ polarizations in these rare $b$-hadron decays. In light of the current experimental situation on lepton flavor universality in rare semileptonic $B$ decays, we discuss the complementary short-distance physics information carried by the $\tau$ polarizations and suggest suitable theoretically clean observables in the form of single- and double-$\tau$ polarization asymmetries.

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• New Physics Search at the CEPC: a General Perspective
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• The fate of $V_1$ vector leptoquarks: the impact of future flavour data
• Electroweak Penguin Decays of $b$-Flavoured Hadrons
• Testing Lepton Flavour Universality in ${\rm Y}(4S)$ Decays
• Circular and Linear ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ Colliders: Another Story of Complementarity
• Precision Tests in $b\to s{\ell }^{+}{\ell }^{-}$ ($\ell =e,\mu$) at FCC-ee
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