The recent developments in the determination of |Vud|, |Vus| and |Vus/Vud| hint towards tensions between three different determinations of the Cabibbo angle, the so-called Cabibbo angle anomalies. The first Cabibbo angle anomaly can be identified as the deficit in the CKM first row unitarity relation. Another tension in the value of the Cabibbo angle is also reported at 3σ level, between two determinations of Vus obtained from semi-leptonic and leptonic kaon decays. A vector-like quark charged as a doublet of SU(2)L can account for this discrepancy. In general, explanations for the CKM unitarity deficit can be in conflict with other observables, such as electroweak observables. I will show that the vector-like doublet of quarks emerges as a favoured candidate for explaining the Cabibbo angle anomalies, particularly if the CDF-II result for mW is taken into consideration. I will review the present status of the anomalies and discuss some possible new physics explanations, in particular mixing of SM quarks with vector-like quarks, up or down type weak singlets or doublets, or the introduction of the gauge horizontal family symmetry acting between the lepton families. I will illustrate other phenomenological consequences of the presence of the vector-like doublet, as flavour changing neutral currents, CP-violation and other effects affecting rare processes.
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Benedetta Belfatto
Institut Fur Theoretische Teilchenphysik, Karlsruhe
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