Global electroweak fit and vector-like leptons in light of the Cabibbo angle anomaly

The “Cabibbo Angle Anomaly” (CAA) originates from the disagreement between the CKM elements Vud and Vus extracted from superallowed beta and kaon decays, respectively, once compared via CKM unitarity. It points towards new physics with a significance of up to 4 σ, depending on the theoretical input used, and can be explained through modified W couplings to leptons. In this context, vector-like leptons (VLLs) are prime candidates for a corresponding UV completion since they can affect Wℓν couplings at tree-level, such that this modification can have the dominant phenomenological impact. In order to consistently assess agreement data, a global fit is necessary which we perform for gauge-invariant dimension-6 operators and all patterns obtained for the six possible representations (under the SM gauge group) of VLLs. We find that even in the lepton flavour universal case, including the measurements of the CKM elements Vus and Vud into the electroweak fit has a relevant impact, shifting the best fit point significantly. Concerning the VLLs we discuss the bounds from charged lepton flavour violating processes and observe that a single representation cannot describe experimental data significantly better than the SM hypothesis. However, allowing for several representations of VLLs at the same time, we find that the simple scenario in which N couples to electrons via the Higgs and Σ1 couples to muons not only explains the CAA but also improves the rest of the electroweak fit in such a way that its best fit point is preferred by more than 4 σ with respect to the begin.

Studying flavor-changing neutral tqZ couplings: Current constraints and future prospects

Possible nonstandard tuZ and tcZ interactions, which induce flavor-changing neutral-current decays of the top quark, are studied in the effective-Lagrangian framework. The corresponding Lagrangian consists of four kinds of nonstandard couplings coming from [Formula: see text] invariant dimension-6 effective operators. The four coupling constants in each interaction are treated as complex numbers independent of each other and constraints on them are derived by using the present experimental limits of the branching fractions for [Formula: see text] and [Formula: see text] processes. Future improvements of those constraints are also discussed as well as possibilities of measurements of these couplings at the High-Luminosity Large Hadron Collider.

INVESTIGATIONS IN THE LEE–WICK ELECTRODYNAMICS

We consider the Lee–Wick (LW) electrodynamics, i.e. the U(1) gauge theory where a (gauge-invariant) dimension-6 operator containing higher derivatives is added to the free Lagrangian of the U(1) sector. A quantum bound on the LW heavy particle mass is then estimated by computing the anomalous electron–magnetic moment in the context of the aforementioned model. This limit is not only within the allowed range estimated by LW, it is also of the same order as that considered in early investigations on the possible effects of the LW heavy particle in e-e+ elastic scattering. A comparative study between the LW and the Coulomb potentials is also done.

CONTRIBUTION OF DIMENSION-SIX BOSONIC OPERATORS TO H→γγ AT ONE-LOOP LEVEL

The decay process H→γγ is examined in a model-independent way within the effective Lagrangian approach. Contribution of a set of irreducible one-loop diagrams involving SU (2)× U (1) invariant dimension-six bosonic operators is evaluated explicitly. The calculation is intended to fill the gap that exists in the current literature on the subject.