Abstract
We consider measurements of exclusive rare semi-tauonic b-hadron decays, mediated by the b rightarrow 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 Zrightarrow bbar{b}, opens the door for the experimental determination of tau polarizations in these rare b-hadron decays. In the 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.
Highlights
Rareleptonic b-hadron decays allow for some of the most sensitive tests of the standard model (SM) description of flavor
In the 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 τ polarizations and suggest suitable theoretically clean observables in the form of single- and double-τ polarization asymmetries
In the present analysis we focus on the potentialities of a Future Circular Collider, and more its
Summary
Rare (semi)leptonic b-hadron decays allow for some of the most sensitive tests of the standard model (SM) description of flavor. The high luminosity of the FCC-ee machine complemented by an excellent vertexing system of the FCC-ee detectors under consideration would allow for unique b-hadron rare decay studies, for instance those involving final-state tau leptons. Polarizations of final-state taus can in principle be reconstructed through their hadronic decay kinematics This in turn allows one to construct a new class of observables, with complementary sensitivities to shortdistance physics, as it will become clear later in the text. About a thousand of events, cleanly reconstructed, can be expected, opening the way to measurements of the angular properties of the decay To our knowledge, these FCC-ee performances are unequaled at any current or foreseeable experiment. The fully charged 5-body decays are relevant as well for the partial reconstruction technique, providing an additional 20% statistics to the baseline study
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