Leptonic Operators for Cabbibo Angle Anomaly with SMEFT RG Evolution

Abstract

The measurements of the Cabibbo--Kobayashi--Maskawa (CKM) elements can be contaminated by new-physics effects. We point out that purely leptonic operators at the high scale can influence semileptonic $K$ decays and nuclear beta decay through renormalization group (RG) running, and hence can influence the measurements of $V_{us}$. Interestingly, through this mechanism, a single six-dimensional effective operator $O_{\ell\ell}$ at the high scale can alleviate the tension due to the Cabibbo angle anomaly, by generating the desired operators at the low scale through RG running. When generated as a result of a $Z'$ model, the non-universal leptonic couplings of this operator can also contribute to the lepton flavor universality violating ratios such as $R_{K^{(*)}}$, which would act as stringent constraints on such scenarios. By performing a global fit of the $Z'$ model, we find that it is essential to have non-universal couplings of such a $Z'$ boson to all three generations of leptons.