Abstract
Flavour- and CP-violating electromagnetic or chromomagnetic dipole operators in the quark sector are generated in a large class of new physics models and are strongly constrained by measurements of the neutron electric dipole moment and observables sensitive to flavour-changing neutral currents, such as the B{,rightarrow ,} X_sgamma branching ratio and epsilon '/epsilon . After a model-independent discussion of the relevant constraints, we analyze these effects in models with partial compositeness, where the quarks get their masses by mixing with vector-like composite fermions. These scenarios can be seen as the low-energy limit of composite Higgs or warped extra dimensional models. We study different choices for the electroweak representations of the composite fermions motivated by electroweak precision tests as well as different flavour structures, including flavour anarchy and U(3)^3 or U(2)^3 flavour symmetries in the strong sector. In models with “wrong-chirality” Yukawa couplings, we find a strong bound from the neutron electric dipole moment, irrespective of the flavour structure. In the case of flavour anarchy, we also find strong bounds from flavour-violating dipoles, while these constraints are mild in the flavour-symmetric models.
Highlights
In all these models, indirect constraints from low-energy precision observables play a crucial role
Since the flavour anarchic model is plagued by strong constraints from dipole operators and from meson–antimeson mixing induced at tree level, it has been suggested that the strong sector is invariant under a flavour symmetry that is only broken by the composite–elementary mixings of one chirality
Dipole operators with quarks and an on-shell photon or gluon are generated at the one-loop level in theories based on the mechanism of partial compositeness, where the quarks get their masses by mixing with heavy vector-like “composite” fermions
Summary
Indirect constraints from low-energy precision observables play a crucial role. The aim of this work is to exploit the computational simplicity of the 4D models to study the impact of different choices for the fermion representations and of different flavour symmetries on the constraints from observables sensitive to dipole operators. To this end, we will use a generalisation of the framework of Ref. At leading order in v/m Q,R, only the Yukawa couplings Y (and not Y ) enter the mass matrix, which is why the latter are sometimes called “wrong-chirality” Yukawa couplings They are not necessary for the generation of quark masses, they are present in many models, and we will see that they play a crucial role in the generation of dipole operators, so we keep them in our Lagrangians. Before discussing the observables probing these operators in turn, we briefly summarise the QCD evolution that is necessary to relate the operators generated at a high new physics scale to the low-energy observables
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