Anatomy of flavour-changing Z couplings in models with partial compositeness

Abstract

In models with partially composite quarks, like composite Higgs models or models with a warped extra dimension, the couplings of quarks to the Z boson generically receive non-universal corrections that are not only constrained by electroweak precision tests but also lead to flavour-changing neutral currents at tree level. The impact of these flavour-changing couplings on rare K and B decays is studied in two-site models for three scenarios: an anarchic strong sector with two different choices of fermion representations both leading to a custodial protection of the $Z\to b\overline{b}$ coupling, and for a strong sector invariant under a U(2)³ flavour symmetry. In the complete numerical analysis, all relevant constraints from ΔF = 2 processes are taken into account. In all scenarios, visible effects in rare K and B decays like $K\to \pi \nu \overline{\nu}$, B _s → μ ⁺ μ ⁻ and B → K ^∗ μ ⁺ μ ⁻ are possible that can be scrutinized experimentally in the near future. Characteristic correlations between observables allow to distinguish the different cases. To sample the large parameter space of the anarchic models, a new method is presented that allows larger statistics than conventional approaches.