The role of non-universal $Z$ couplings in explaining the $V_{us}$ anomaly

The tension among measurements of $V_{us}$ from different channels, the so-called Cabibbo Angle Anomaly, can be interpreted as a signal of lepton flavor universality (LFU) violation in the $W$ boson couplings. We investigate this issue in the framework of effective field theory, keeping the gauge structure of the Standard Model (SM) unchanged. We introduce gauge-invariant dimension-6 effective operators that couple the Higgs doublet to leptons, thereby giving non-universal tree-level contributions to the couplings of electroweak gauge bosons. Due to the $SU(2)_L$ gauge symmetry, a tension arises between the $V_{us}$ measurements that are affected by new $W$ couplings, and the electroweak precision measurements, which are also affected by the new $Z$ couplings. We show that this tension can be alleviated by allowing additional sources of gauge-invariant couplings of $Z$ boson to left- or right-handed leptons, and find the optimal regions indicated by the current data in the Wilson-coefficient space. We illustrate our model-independent results with the examples of minimal extensions of the SM involving the vector-like lepton (VLL) models, among which the model with a $SU(2)_L$ triplet $X$ with hypercharge $Y=-1$ comes close to the optimal solution. We point out that dimension-6 operators coupling the Higgs doublet to leptons can affect the rate of $h \to \tau \tau$ decay significantly in general, however this effect is restricted to less than a per cent level for the minimal VLL models.