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

TL;DR

This paper investigates how partial compositeness in quark models leads to flavor-changing Z couplings, affecting rare K and B decays, with detailed numerical analysis and new sampling methods for parameter space.

Contribution

It provides a comprehensive analysis of flavor-changing Z couplings in partial compositeness models, including novel numerical techniques and predictions for observable rare decays.

Findings

Possible observable effects in rare K and B decays

Distinct correlations between decay observables

Constraints from Delta(F)=2 processes are significant

Abstract

In models with partially composite quarks, 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->bb coupling, and for a strong sector invariant under a U(2)^3 flavour symmetry. In the complete numerical analysis, all relevant constraints from Delta(F)=2 processes are taken into account. In all scenarios, visible effects in rare K and B decays like K->pi nu anti-nu, B(s)->mu+mu- and B->K*mu+mu- 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.