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

Abstract

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.

Global analysis of leptophilic Z′ bosons

“…Such scenarios are particularly interesting as within recent years several hints for the violation of lepton flavour universality (LFU) have been acquired. These include τ → µνν/τ → eνν, τ → µνν/µ → eνν [54,55] and the Cabibbo angle anomaly [56][57][58][59], which can also be interpreted as a sign of LFUV [60][61][62][63][64][65][66][67][68][69][70][71]. Furthermore, even though in this case also (small) couplings to bottom and strange quarks are necessary, Z bosons are among the prime candidates for explaining the discrepancies between the SM predictions and data in b → s + − transitions [12,34,51,53,67,.…”

The singly-charged scalar singlet as the origin of neutrino masses

“…Another observable which has recently attracted attention (see for instance [83][84][85][86][87][88][89][90]) and is of interest for the scenario under consideration is the sum of the squares of the absolute values of the first-row elements of the Cabibbo-Kobayashi-Maskawa (CKM) matrix:…”

Excited lepton triplet contribution to electroweak observables at one loop level

“…However, the exotic leptons can mix with the SM Higgs boson [5] giving rise to a small splitting of the masses of the mutiplet members. Also, these couplings will give rise to dimension six operators with contribution to the SM observables [6,7] smaller than the dimension 5 operators, arising from compositeness, under consideration in the present work.…”

“…Like in other models with exotic fermions [5] couplings to SM Higgs may be present. However, these interactions [6,7] result in weaker effects on the SM observables than the ones introduced through compositeness.…”