학술논문
Global Fit of Modified Quark Couplings to EW Gauge Bosons and Vector-Like Quarks in Light of the Cabibbo Angle Anomaly
Document Type
Working Paper
Source
JHEP03(2023)234
Subject
Language
Abstract
There are two tensions related to the Cabibbo angle of the CKM matrix. First, the determinations of $V_{us}$ from $K_{\mu 2}$, $K_{\ell3}$, and $\tau$ decays disagree at the $3\sigma$ level. Second, using the average of these results in combination with $\beta$ decays (including super-allowed $\beta$ decays and neutron decay), a deficit in first-row CKM unitarity with a significance of again about $3\sigma$ is found. These discrepancies, known as the Cabibbo Angle anomaly, can in principle be solved by modifications of $W$ boson couplings to quarks. However, due to $SU(2)_L$ invariance, $Z$ couplings to quarks are also modified and flavour changing neutral currents can occur. In order to consistently assess the agreement of a new physics hypothesis with data, we perform a combined analysis for all dimension-six Standard Model Effective Field Theory operators that generate modified $W$ couplings to first and second generation quarks. We then study models with vector-like quarks, which are prime candidates for a corresponding UV completion as they can affect $W$-quark couplings at tree level, and we perform a global fit including flavour observables (in particular loop effects in $\Delta F=2$ processes). We find that the best fit can be obtained for the $SU(2)_L$ doublet vector-like quark $Q$ as it can generate right-handed $W$-$u$-$d$ and $W$-$u$-$s$ couplings as preferred by data.
Comment: 46 pages, 16 figures. v2: Added references, minor numerical changes due to bug in code, conclusions unchanged. v3: journal version, minor clarifications added
Comment: 46 pages, 16 figures. v2: Added references, minor numerical changes due to bug in code, conclusions unchanged. v3: journal version, minor clarifications added